RIMERS, 


H 


UXLEY- 


R« 


Y 


A.  GEIKIE. 


UNIVERSITY  OF  CALIFORNIA 
AT   LOS  ANGELES 


GIFT  OF 

Pacific    .'lectric   Co, 


LOCKYER'S   ASTRONOMY. 

ELEMENTS  OF  ASTRONOMY: 

Accompanied  with   numerous    Illustrations,  a  Colored   Repre- 
sentation of  the  Solar,  Stellar,  and  Nebular  Spectra, 
and   Celestial  Charts   ot   the  Northern 
and' the  Southern  Hemisphere. 

By  J.  NORMAN  LOCKYER. 

American  edition,  revised  and  specially  adapted  to  the  Schools 
of  the  United  States. 

•Lzmo.     -p.?.  pages.     Price,  $1.50. 

The  volume  is  as  practical  as  possible.  To  aid  the  student 
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D.  APPLETON  &  CO.,  PUBLISHERS, 

540  &  5 ;  i_  BROAD  WAY.  NEW  YORK. 


The  RALPH  D.  REED  LIBRARY 

i.NT  OF  GEOLOGY 

VERITY  of  CALIFORNIA 

LOS  ANGELES,  CALIF. 


SCIENCE     PRIMERS,  edited  by 

PROFESSORS    HUXLEY,     RoscoE,    and 
BALFOUR    STEWART. 

V. 
GEOLOGY. 


REED 


Srimte 


GEOLOGY. 


ARCHIBALD    GEIKIE,    LL.D.,    F.R.S., 

Director  of  the  Geological  Survey  of  Scotland,  and  Murchison-Professof 
of  Geology  and  Mineralogy  in  the  University  oj  Edinburgh. 


WITH    ILLUSTRATIONS. 


NEW  YORK: 
D.    APPLETON   AND   COMPANY, 

549    &   551   BROADWAY. 
18V9. 


CONTENTS. 

ART.  PAGE 

INTRODJCTORY i — 14  i 

DIFFERENT  KINDS  OF  STONES    .     .    .  '  .  15 — 31  6 

WHAT  STONES  HAVE  TO  TELL  us  ...  32—42  14 
SEDIMENTARY  ROCKS  : — 

I.   What  Sediment  is 43 — 56  19 

ii.   How  Gravel,   Sand,   and  Mud    are 

made    .     .     . 57— 76  23 

in.  How  Gravel,  Sand,  and  Mud  be- 
come Sedimentary  Rocks  ...  77  — 105  32 

IV.  How  the  Remains  of  Plants  and 
Animals  come  to  be  found  in 
Sedimentary  Rocks 106—117  44 

v.    A  Quarry  and  its  Lessons  .     .     .     .      118—131  50 

ORGANIC  ROCKS,  OR  ROCKS  FORMED  OF 
THE  REMAINS  OF  PLANTS  AND  ANI- 
MALS : — 

i.    Rocks   formed   of   the   Remains    of 

Plants 132 — 152  56 

ii.   Rocks   formed   of  the   Remains    of 

Animals 153—162  66 

311989 


CONTENTS. 


IGNEOUS  ROCKS: — 

I.    What  Igneous  Rocks  are    ....  163 — 175  74 

II.    Where,  Igneous  Rocks  come  from    .  176 — 187  83 

THE  CRUST  OF  THE  EARTH: — 

I.    Proofs  that  parts  of  the  Crust  have 

been  pushed  up 188 — 206          91 

II.    Proofs  that  parts  of  the  Crust  have 

sunk  down 207 — 217         loo 

in.    Proofs  that  the  Rocks  of  the  Earth's 
Crust  have  been  tilted,  crumpled, 

and  broken 218 — 225         106 

IV.   The  Origin  of  Mountains  ....  227 — 242         1 1 2 
v.    How  the  Rocks  of  the  Crust  tell  the 

History  of  the  Earth    ....  243 — 254         120 

CONCLUSION •.    .  255 — 258        127 


SCIENCE    PRIMERS. 

GEOLOGY. 


INTRODUCTORY. 

i.  AN  ordinary  dwelling-house,  such  as  those  in 
which  most  of  us  live,  is  built  of  various  materials,  and 
one  of  these  is  always  stone .  In  the  walls,  the  hearths, 
the  chimney-pieces,  and  the  roofs,  stone  is  used.  But 
in  each  of  these  cases  the  kind  of  stone  usually  differs 
from  that  employed  in  the  rest  of  the  building.  Thus 
the  walls  may  be  made  of  freestone,  or  limestone,  or 
brick,  the  hearths  of  flagstone,  the  roofs  of  slate  or 
tiles,  the  chimney-pieces  of  marble,  while  still  another 
sort  of  stone  called  coal  is  burnt  in  the  fireplaces. 
Go  out  into  the  streets  and  you  find  a  still  greater 
diversity.  The  causeway-stones  are  of  one  kind, 
those  of  the  foot-pavement  of  another.  Many  dif- 
ferent ornamental  varieties  are  made  use  of  in  the 
shops  and  buildings.  So  that  merely  by  looking  at 
houses  and  streets  you  may  readily  perceive  that 
there  are  many  different  kinds  of  stone. 


2  SCIENCE  PRIMERS.       [INTRODUCTORY. 

2.  If  you  examine  them  a  little  more  narrowly  you 
will  see  that  they  receive  various  treatment  before  they 
become  part  of  a  building.     The  stones  of  the  walls 
have   been   chipped   and   dressed   with  chisels   and 
hammers  ;  the  marble  of  the  chimney-pieces  has  been 
smoothed  and  polished  ;   the  slates  have  been  split 
into  thin  plates.     But  some  of  these  building  mate- 
rials have  undergone  much   greater  changes.      The 
bricks,  for  instance,  were  originally  soft  clay  which 
has  been  hardened  by  being  baked  in  ovens.     The 
mortar  by  which  the  stones   or  bricks  of  the  walls 
are   held    together   has   been     obtained  by   burning 
limestone  in  kilns.     The  iron  used  in  the  house  was 
first  of  all  in  the  state  of  a  dull  red  or  brown  stone, 
which  had  to  be  roasted  and  melted  before  the  clear 
bright  metal  came  out  of  it. 

3.  But  although  these  various  stones  differ  so  much 
from    each    other,    they  agree    in  one   point — they 
come   from  underneath   the    surface    of  the 
ground.     If  you  could  trace  back  each  of  them  to 
the  place  from  which  it  came  you  would  find  that  the 
freestone  and  limestone  were  taken  out  of  quarries, 
perhaps  not  very  far  away,  that  the  slates  were  cut 
out  of  the  side  of  some  hill,  probably  in  Wales,  that 
the  marble  was  quarried  out  of  some  distant  moun- 
tain, possibly  in  Italy,  that  the  coal  was  dug  out  of 
mines,  sunk  deep  into  the  earth  in  some  part  of  Bri- 
tain, and  that  the  bricks  were  made  from  clay  dug  out 
of  pits  on  some  low  ground  in  your  neighbourhood. 

4.  In  this  country  the  greater  part  of  the  surface 
has  a  green  grassy  covering  even  over  the  sides  of 
the  hills — cornfields,  meadows,  woods,  and  orchards 
spread  over  it,  concealing  what  lies  beneath  them,  as  a 


INTRODUCTORY.]  GEOLOGY. 


carpet  conceals  a  floor.  But  this  mantle  of  vegetation 
with  the  soil  on  which  it  grows  is  only  a  thin  coating. 
You  can  easily  dig  through  the  grass  and  soil,  or,  better 
still,  you  can  watch  their  removal  in  quarries,  pits,  or 
excavations  of  any  kind.  You  find  them  to  form  a 
mere  outer  layer  only  a  few  feet  thick  at  the  most. 
Underneath  them  there  always  lies  some  kind  of 
stone.  So  that  just  as  in  pulling  up  the  carpet  of  a 
room  you  lay  bare  a  wooden  floor,  so  in  peeling  off 
the  outer  skin  of  vegetation  and  soil  from  any  part 
of  the  land  you  expose  a  stone  floor. 

5.  On  this  floor  of  stone  we  are  walking  every  day 
of  our  lives.     It  stretches  all  over  the  globe,  forming 
the  bottom  of  the  sea  and  the  surface  of  the  land. 
Unlike  the  floors  of  our  houses  it  is  very  uneven,  as 
you  well  know.     In  some  places  it  spreads  out  into 
wide  flat  plains,  elsewhere  it  shoots  up  into  high  and 
rugged  mountains. 

6.  Again,  this  vast  world-wide  floor  differs  from  our 
little  wooden  floors  in   the  wonderful  variety  of  its 
materials.     You  see  only  a  small  part  of  this  variety 
in    the    various    stones  we  use  in   building.     There 
is  an  almost    endless    number  of    other  stones.     A 
builder  is  content   if  he  can  get  his  floors  made  of 
one  uniform  sort  of  wood  which  will  last.     But  the 
great    stone  floor   on  which  we    are   living   has  no 
such  uniformity.     Its    varied  materials  are  grouped 
together  in  the  most  irregular  and  changing  manner, 
insomuch  that   if  you  made   a  map  of  them  all,  it 
would  be  like    the  intricate  pattern  of  some  costly 
carpet. 

7.  It  is  this  stone  floor  of  which  I  wish  to  speak  to 
you  in  the  following  Lessons — what  it  is  made  of  and 


4  SCIENCE  PRIMERS.        [INTRODUCTORY. 

how  its  different  parts  were  put  together.  At  first 
sight,  perhaps,  it  may  seem  to  you  that  there  can 
be  nothing  very  interesting  or  attractive  about  such  a 
subject.  Let  me  show  you  how  it  is  related  to  you 
by  the  following  illustration. 

8.  Take  a  map  of  the    British    Islands  and  draw 
across  it  two  pencilled  lines.     Let  one  of  these  lines 
begin  at  Liverpool  and  stretch  across  England,  touch- 
ing Stafford,  Birmingham,  and  Cambridge,  to  the  sea 
at  Harwich.     Let  the  other  run  across  the  breadth  of 
Scotland  from  the  island  of  Skye  to  Montrose.* 

9.  Suppose  that  two  foreigners  who  had  never  been 
in  this  country  were  to  land  on  the  west  coast,  and  after 
crossing  the  island,  each  along  one  of  the  lines  you 
have  drawn,  were  afterwards  to  meet  again  on  the 
Continent  and  compare  notes  as  to   what  they  had 
seen.     The  traveller  who  journeyed  along  the  line 
from   Liverpool   to    Harwich    might  report  in  some 
such  words  as  these  : — "  I  am  astonished  at  the  flat- 
ness of  Britain.     I  went  across  the  whole  breadth  of 
the   island  and  did  not  see  a  single   undulation  of 
the  ground  worthy  of  the  name  of  a  hill.     Most  of 
the   land   is    wonderfully   fertile,    being  in   one  part 
covered  with  corn-fields,  in  another  with  orchards  or 
woods,  while   wide   tracts  are  given  up   to  pasture. 
The   houses  are  built  of  brick.     I  saw  some  large 
cities  crowded  with  people  and  alive  with  all  kinds  of 
industry.     I  noticed,  too,  that  in  some  parts  of  the 
country  a  great  deal  of  the  wealth  of  the  inhabitants 
came  from  underground.     In  Cheshire  they  bring  up 
large  quantities  of  salt  from  mines.     In  Staffordshire 

*  A  similar  illustration  has  been  used  by  Buckland,  in  his 
Bridgewater  Treatise. 


INTRODUCTORY.]  GEOLOGY. 


they  extract  coal  and  iron  from  numerous  deep  pits. 
But  on  the  whole  Britain  seems  to  me  given  up 
chiefly  to  the  growing  of  corn  and  the  feeding  of 
cattle." 

10.  The  other  traveller  would  have  a  very  different 
story  to  tell.  "  I  cannot  understand,"  he  might  say, 
"  how  you  can  talk  of  Britain  as  in  any  sense  a  flat 
country.  I  too  crossed  the  island  from  sea  to  sea, 
landing  on  the  coast  of  Inverness-shire  and  sailing 
from  the  port  of  Montrose.  But  I  could  see  very 
little  low  or  level  land  the  whole  way.  It  is  one  inter- 
minable succession  of  rough  high  mountains  and 
deep  rocky  valleys.  I  could  see  no  towns,  hardly 
any  villages,  until  I  came  to  the  eastern  coast.  The 
people  live  in  houses  of  stone  ;  I  could  not  see  a 
single  brick  anywhere.  They  have  no  coals  except 
what  are  brought  from  a  distance,  and  most  of  the 
poorer  people  cut  the  peat  on  the  hills  and  use  it  for 
fuel.  I  saw  no  mines  in  my  journey,  and  no  manu- 
factures of  any  kind.  The  population  is  but  scanty, 
and  seems  to  be  occupied  chiefly  with  sheep-farming. 
If  I  might  judge  of  the  whole  of  Britain  from  what 
I  have  seen  with  my  own  eyes  I  would  describe  it  as 
a  rough,  mountainous,  barren  island,  without  com- 
merce or  industry,  and  fit  only  for  pasture-land  or 
grouse-shooting,  and  here  and  there  for  tillage." 

ii.  Now  each  of  these  supposed  travellers  would 
have  given  a  true  enough  account  of  this  country  so 
far  as  his  own  personal  experience  of  it  went.  And 
yet  both  of  them  would  have  been  quite  wrong  in 
supposing  that  what  he  had  seen  to  be  true  of  one 
part  of  the  country  was  true  in  like  manner  of  the 
whole. 


6  SCIENCE  PRIMERS.  [DIFFERENT 

12.  But  why  is  it  that  there  should  be  this  great 
difference   between    different    portions    of    Britain  ? 
What  makes  one  region  mountainous,  another  level, 
one  fertile,  another  barren,  one  crowded  with  people 
and  the   scene  of  all  manner   of   industry,  another 
thinly  peopled  and  given  up  to  the  rearing  of  sheep 
and  the  shooting  of  game  ? 

13.  These  great  differences  of  the  surface 
of  the  country  depend  upon  differences  be- 
tween the  stones  or  rocks. 

14.  Now  you  can  easily  understand  that  if  so  much 
of  the  character  of  a  country  and  of  its  inhabitants 
depends  upon  the  nature  of  the  stones  underneath, 
it  is  very  desirable  that  we  should  know  something 
about  these  stones,  how  they  came  to  be  formed,  what 
they  consist  of,  and  how  it  is  that  they  should  form 
plains  or  low  grounds  in  one  place,  and  single  hills 
or  lofty  mountains  in  another.     This  kind  of  know- 
ledge belongs  to  the  science  of  Geology. 


DIFFERENT    KINDS    OF    STONES. 

15.  If  I  were  to  ask  you  how  many  different  kinds 
of  books  you  have  seen  in  the  course  of  your  lifetime, 
you  would  perhaps  say  that  it  was  quite  impossible  to 
count  them.  You  have  seen  many  that  were  ne\v, 
some  that  were  old  ;  big  books  and  little  books  ;  some 
with  boards,  others  merely  wrapped  up  in  paper ; 
some  beautifully  bound  in  cloth  of  red,  green,  blue, 
or  other  colours,  others  cased  in  leather  and  covered 
with  rich  gilding ;  some  printed  in  large,  others  in 
small  letters ;  some  plentifully  supplied  with  pictures, 


KINDS    OF    STONES.]  GEOLOGY. 


others  without  any  at  all.  In  short,  you  might  go  on 
for  a  long  time  trying  to  count  up  all  these  differ- 
ences among  the  books  which  you  have  met  with. 
But  now  if  you  think  a  moment  you  will  see  that,  after 
all,  these  are  only  outside  differences.  The  really 
important  part  of  the  book  is  not  the  binding,  or  the 
paper,  or  the  printing,  but  the  words  which  the  book 
has  to  make  known  to  you.  You  might  print  these 
words  in  very  small  type  and  make  them  up  into  a  little 
book,  or  in  very  large  and  widely  spaced  type  and 
make  a  big  book  ;  you  might  put  in  pictures  or  leave 
them  out ;  you  might  bind  the  book  in  cloth  or  in 
leather,  or  give  it  no  binding  at  all ;  but  still  it  would 
be  in  reality  the  same  book  all  the  time. 

1 6.  When  you  pass,  then,  from  such  mere  unimpor- 
tant external  resemblances  or  differences  to  what  the 
books  properly  are  in  themselves,  you  soon  discover 
that  after  all  there  are  not  so  many  kinds  as  you  had 
imagined.     You  can  pick  them  out  and  sort  them  into 
groups  according  to  the  subjects  of  which  they  treat. 
Thus  in  your  little  libraries  you  find  that  some   are 
Books  of  Grammar,  others  Books  of  History,  others 
Books  of  Geography,  with   Books  of  Poetry,  Books 
of  Travels,  Books  of  Tales,  and  so  on.     Under  each 
of  these  names  you  could  put,  if  you  had  them,  hun- 
dreds of   books,   resembling   each   other  in  treating 
about  the  same  things,  whether  they  were  old  books 
or  new,  large  or  small,  bound  or  unbound. 

17.  In  arranging  your  books  in  this  way,  not  accord- 
ing to  their  mere  superficial  accidental  resemblances, 
but  according  to  the  subjects  which  they  treat  of  in 
common,  that  is,  their  real  resemblances,  you  would 
follow  what  is  called  a  Principle  of  Ciassifica- 

2 


8  SCIENCE  PRIMERS.  [DIFFERENT 

tion.  It  would  not  matter  how  many  different  books 
came  into  your  hands ;  they  might  be  written,  too, 
in  English,  French,  German,  Latin,  Greek,  or  in  any 
language.  Still,  following  your  principle  of  classifica- 
tion, you  would  be  able  to  arrange  them  all  in  their 
proper  places,  all  the  books  on  the  same  subject 
being  put  together,  so  that  at  any  moment  you  could 
lay  your  hands  on  any  particular  book  which  might  be 
wanted. 

1 8.  Suppose  that  instead  of  books  you  are  asked  to 
arrange  stones  according  to  their  several  kinds.     You 
think  over  the  names  of  all  the  different  stones  you 
know  and  try  to  recollect  their  characters.     Perhaps 
you  begin  by  arranging  them  according  to  colour,  as 
for  instance,  Black  Stones,  such  as  Coal ;  White  Stones, 
such  as  Chalk.     But  in  a  little  time  you  find  that  the 
same  stone,  marble  for  instance,  is  sometimes  black 
and  sometimes  white.     Plainly,  therefore,  colour  will 
not   do  for  your   principle   of    classification  among 
stones,  any  more  than  it  would  do  for  books.     Then 
you  might  go  on  to  see  how  a  grouping  into  Hard 
Stones  and  Soft  Stones  would  do.     But  as  soon  as 
you  begin  this  kind  of  classification,  you  find  that  you 
need  to  put  side  by  side  stones  which  are  so  utterly 
unlike  each  other  that  you  feel  sure  that  mere  hard- 
ness or  softness  is  one  of  those  accidental  or  outside 
characters,  like  the  paper  or  printing  of  a  book. 

19.  You  must  find  out  then  what  are  the  real  and 
essential  characters  of  stones.     Now  how  did  you  do 
this   in  the   case   of  books  ?     You   examined   their 
contents  and  placed  those  together  which  on  reading 
them  you  found  to  be  devoted  to  the  same  subject. 
You  must  follow  the  same  course  with  stones. 


KINDS   OF   STONES.]  GEOLOGY. 


20.  But  you  may  ask,  "  How  are  we  to  read  the 
contents  of  stones  ?     Surely  this  must  be  very  difficult, 
for  is  there  not  an  infinitely  greater  number  of  kinds 
of  stones  than  of  books  ?  "     By  no  means.     You  will 
soon  learn  that  it  is  not  so  difficult  as  you  might  sup- 
pose to  read  the  contents  of  stones,  and  that  in  reality 
the  chief  groups  of  stones  are  very  much  fewer  in 
number  than  the  chief  groups  of  books.     Let  us  see. 

21.  Here  are  three  pieces  of  stone  : — 

1.  A  piece  of  Sandstone. 

2.  A  piece  of  Granite. 

3.  A  piece  of  Chalk. 

2  2.  You  are  quite  familiar  with  each  of  these  kinds 
of  stone.  Sandstone  is  a  common  material  for  walls, 
lintels,  hearths,  and  flagstones.  Granite  may  now  be 


FIG.  i.— Piece  of  Sandstone. 

frequently  seen  in  polished  columns  and  slabs  in 
public  buildings,  shops,  and  in  tombstones ;  and  the 
streets  in  many  of  our  large  cities  and  towns  are  now 
paved  with  it.  Common  white  Chalk  is  well  known 
to  everybody. 

23.  Take  the  piece  of  sandstone  in  your  hands  and 


io  SCIENCE  PRIMERS.  [DIFFERENT 

examine  it  carefully,  using  even  a  magnifying  glass  if 
the  grains  are  minute.  Then  write  down  each  of  the 
characters  you  observe  one  after  another.  You  will 
of  course  pay  little  heed  to  the  colour,  for  sandstones, 
like  books,  may  be  red  or  white,  green  or  yellow,  or 
indeed  of  almost  any  colour.  Nor  will  you  give 
much  weight  to  the  hardness  or  softness  as  an  essen- 
tial character,  for  you  may  find  even  in  a  small  piece 
of  the  stone  that  one  part  is  quite  hard  while  a  neigh- 
bouring place  is  soft  and  crumbling. 

24.  If  your  piece  of  sandstone  has  been  well  chosen 
for  you,  you  will  be  able  to  write  down  the  following 
characters : — 

(i.)  The  stone  is  made  up  of  small  grains. 

(2.)  The  grains  are  all  more  or  less  rounded  or  worn. 

(3.)  By  scraping  the  surface  of  the  stone  these 
rounded  grains  can  be  separated  from  the  stone,  and 
when  they  lie  in  this  loose  state  they  are  seen  to  be 
mere  grains  of  sand. 

(4.)  More  careful  examination  of  the  stone  shows 
that  the  grains  tend  to  lie  in  lines,  and  that  these 
lines  run  in  a  general  way  parallel  with  each  other. 

(5.)  The  grains  diifer  from  each  other  in  size  and  in 
the  material  of  which  they  are  made.  Most  of  them 
consist  of  a  very  hard  white  or  colourless  substance  like 
glass,  some  are  perhaps  small  spangles  of  a  material 
which  glistens  like  silver,  others  are  softer  and  of 
various  colours.  They  lie  touching  each  other  in 
some  sandstones ;  in  others  they  are  separated  by  a 
hard  kind  of  cement  which  binds  them  all  into  a 
solid  stone.  It  is  this  cement  which  usually  colours 
the  sandstone,  since  it  is  often  red  or  yellow,  and 
sometimes  green,  brown,  purple,  and  even  black. 


KINDS   OF   STONES.]  GEOLOGY. 


25.  Summing  up  these  characters  in  a  short  defini- 
tion, you  might  describe  your  sandstone  as  a  stone 
composed  of  worn,  rounded  grains  of  various 
other  stones  arranged  in  layers. 

26.  Proceed  now  in  the  same  way  with  the  piece 
of  Granite.     You  find  at  once  a  very  different  set  of 
appearances,  but  after  a  little  time  you  will  be  able  to 
make  out  and  to  write  down  the  following  : — 


FIG.  2. — Piece  of  Gi 


(i.)  The  stone  contains  no  rounded  grains. 

(2.)  It  is  composed  of  three  different  substances, 
each  of  which  has  a  peculiar  crystalline  form  (see 
Chemistry  Primer,  Art.  23).  Thus,  one  of  these, 
called  Felspar,  lies  in  long  smooth-faced,  sharply 
defined  crystals  of  a  pale  flesh  colour,  or  dull  white, 
which  you  can  with  some  difficulty  scratch  with  the 
point  of  a  knife.  These  are  the  long  white  sharp- 
edged  objects  shown  in  the  drawing  (Fig  2).  Another, 
termed  Mica,  lies  in  bright  glistening  plates  which 
you  can  easily  scratch  and  split  up  into  thin  trans- 
parent leaves.  If  you  compare  these  shining  plates 
with  the  little  silvery  spangles  in  the  sandstone  you 
will  see  that  they  are  the  same  material.  The  third, 
named  Quartz,  is  a  very  hard,  clear,  glassy  substance 


12  SCIENCE  PRIMERS.  [DIFFERENT 

on  which  your  knife  makes  no  impression,  but  which 
you  may  recognize  as  the  same  material  out  of  which 
most  of  the  grains  of  the  sandstone  are  made. 

(3.)  The  crystals  in  granite  do  not  occur  in  any 
definite  order,  but  are  scattered  at  random  through 
the  whole  of  the  stone. 

27.  Here  are   characters  strikingly  different  from 
those   of  the  sandstone.      You  might  make  out  of 
them   such  a   short  definition  as  this — Granite  is  a 
stone  composed  of  distinct  crystals  not  laid 
down  in  layers  but  irregularly  interlaced  with 
each  other. 

28.  Lastly  go  through  the  same  process  of  examina- 
tion with  your  piece  of  Chalk.     At  first  sight  this  stone 
seems  to  have  no  distinct  characters  at  all.     It  is  a 
soft,   white,   crumbling   substance,  soils  your  fingers 
when  you  touch  it,  and  seems  neither  to  have  grains 
like  the  sandstone  nor  crystals  like  the  granite.     You 
will  need  to  use  a  magnifying  glass,  or  even  perhaps  a 
microscope,  to  see  what  the  real  nature  of  chalk  is. 
Take  a  fine  brush  and  nib  off  a  little  chalk  into  a 
glass  of  clear  water ;  then  shake  the  water  gently  and 
let  it  stand  for  a  while  until  you  see  a  layer  of  sedi- 
ment on  the  bottom.     Pour  off  the  water  and  place  a 
little  of  this  sediment  upon  a  piece  of  glass,  and  look 
at  it  under  the  microscope  or  magnifying-glass.     You 
will  find  it  to  have  strongly  marked  characters,  which 
might  b£  set  down  thus  : — 

(i.)  The  stone,  though  it  seems  to  the  eye  much 
more  uniform  in  its  texture  than  either  sandstone  or 
granite,  is  made  up  of  particles  resembling  each  other 
in  colour  and  composition,  but  presenting  a  variety 
of  forms. 


KINDS  OF  STONES.]         GEOLOGY.  13 

(2.)  It  consists  of  minute  shells,  pieces  of  coral, 
fragments  of  sponges,  and  white  particles,  which  are 
evidently  the  broken-down  remains  of  shells.  In 
Fig.  3  you  see  some  of  these  chalk  grains  as  they 
appear  when  you  place  them  under  a  microscope  which 


FIG.  3. — Some  of  the  Grains  of  a  piece  of  Chalk. 

magnifies  them  fifty  times.  Larger  and  well-preserved 
shells,  sea-urchins,  and  remains  of  other  sea-creatures 
occur  imbedded  in  the  chalk.  (See  Fig.  23.) 

29.  As  a  brief  description  of  chalk  you  might  say 
that  it  is  a  stone  formed    out  of  the    remains 
of  once-living  animals. 

30.  You  should  repeat   this  kind  of  examination 
again  and  again  until  you  get  quite  familiar  with  the 
characters  which  have  been  written  down  here.     And 
you  will  see  why  it  is  important  for  you  to  do  so  when 
you   come   afterwards   to  find   out  that  these  three 
stones   are  examples  of  the  three  great  groups  into 
which  most  of   the  rocks  of  the  world  may  be  ar- 
ranged.    So  that  when  you  master  the  composition  of 
a  piece  of  sandstone,  or  chalk,  or  granite,  and  learn 
how  each  stone  was  formed,  you  not  only  do  that,  but 
lay  a  foundation  of  knowledge  which  will  enable  you 
to  understand  how   by  far  the  greater  part  of   the 
stones  of  our  mountains,  valleys,  and  sea-shores  came 
into  existence. 


14  SCIENCE  PRIMERS.       [WHAT  STONES 

31.  In  spite  then  of  the  apparently  infinite  diversity 
of  the  stones  of  which  the  globe  is  built  up,  you  see 
that  by  a  little  study  they  may  be  grouped  into  very 
few  classes.  You  have  to  follow  a  simple  prin- 
ciple of  classification,  and  each  stone  you  may  meet 
with  falls  naturally  into  its  own  proper  group.  You 
do  not  concern  yourselves  much  with  mere  outer 
shape  and  hue,  but  try  to  find  out  what  the  stone  is 
made  of,  and  ask  whether  it  should  be  placed  in 
the  Sandstone  group,  or  in  the  Granite  group,  or  in 
the  Chalk  group. 


WHAT    STONES    HAVE    TO    TELL    US. 

32.  But  if  you  went  no  further  than  merely  being 
able  to  arrange  stones  under  their  proper  divisions  it 
would  be  hardly  worth  your  while  to  study  them  at 
all.  You  would  be  like  people  who  could  put  a 
library  into  such  excellent  order  that  every  volume 
should  stand  on  its  proper  shelf  and  compartment, 
ready  for  easy  reference  at  any  moment,  but  who 
should  rest  content  with  this  mere  systematic  arrange- 
ment and  never  open  any  of  the  books  to  make 
themselves  acquainted  with  the  contents  as  well  as 
with  the  boards.  The  classification  of  stones,  or 
flowers,  or  birds,  or  fishes,  or  any  other  objects  in 
nature,  is  in  itself  of  no  more  service  than  such  an 
arranging  of  a  library,  unless  you  use  it  in  helping 
you  to  understand  better  what  is  the  nature  of  the 
things  you  classify  and  how  they  are  related  to  each 
other. 


TELL  us.]  GEOLOGY.  15 

33.  This  habit  of  classifying  what  we  discover  lies 
at  the  base  of  all  true  science.     Without  it  we  could 
not  make  progress  ;  we  should  always  be  in  a  maze, 
and  would  never  know  what  to  make  of  each  new 
thing  we  might  find  out.     We  should  be  like  people 
turned  into  a  great  hall    and    required    to    educate 
themselves  there,  with  the  floors  and  galleries  strewed 
all  over  with  piles  of  books  in  all  languages  and  on 
every  subject,  but  utterly   and  hopelessly  in    confu- 
sion. 

34.  Let  us  now  try  what  this  habit  can  do  for  us 
among  the  seemingly  endless  variety  of  stones  with 
which  the  world  is  stored. 

35.  We  take  again  our  three  pieces  of  stone — sand- 
stone, chalk,  and  granite — and  compare  other  stones 
with  them.     We  get  out  of  town  to  the  nearest  pit 
or  quarry  or   ravine,  to  any  opening   in   fact,  either 
natural  or  artificial,  which  will  enable  us  to  see  down 
beneath  the  grass  and  the  soil  of   the    surface.     In 
one  place  we  may  find  a  clay-pit,  in  another  a  sand- 
stone-quarry, in  another    a    railway    cutting   through 
chalk   or    limestone,    in   another    a    deep    ravine    in 
hard  rocks  with  a  stream,  flowing  at  its  bottom.     It 
does  not    matter    for  our  present  purpose  what  the 
nature  of  the  opening  be,  provided  it  shows  us  what 
lies   beneath  the    soil.     In  all  such  places  we  meet 
with  stone  of  some  kind,  or  of  many  different  kinds. 
By  a  little  practice  we  learn  that  these  various  sorts 
of    stones    may   be   usually   arranged   under  one  or 
other    of    the     three    divisions    of    which    mention 
was    made  in  last   Lesson.       For    example,    a    large 
number  of    stones  will    be    found  answering  to  the 
general    description  which  you  found  to  be    true  of 


1 6  SCIENCE  PRIMERS.       [WHAT  STONES 

Sandstone.  These  will  of  course  be  placed  to- 
gether with  our  piece  of  sandstone.  Another  con- 
siderable quantity  of  stones  will  be  met  with  made 
up  wholly  or  almost  wholly  of  the  remains  of  plants 
or  of  animals.  These  we  arrange  in  the  same 
division  with  our  piece  of  chalk.  Lastly,  a  good 
many  stones  may  be  met  with  built  up  of  crystals  of 
different  kinds,  and  these,  for  the  present,  we  class 
together  with  our  piece  of  granite. 

36.  In  this  way  you  would  advance  from  the  mere 
pieces  of  stone  which  you  can  hold  in  your  hand,  up  to 
the  masses  of  stone  lying  under  a  whole  parish  or  a 
county   or   even   the   entire   kingdom.     You    would 
learn  that  a  long  range  of  hills,  stretching  completely 
across  England    from  the  coasts  of    Dorsetshire    to 
those    of   Yorkshire,  is    formed    of   chalk,  and    that 
other  parts  of  the  country  lie  upon  kinds  of  stone  in 
many  respects  resembling  chalk.      You  would  soon 
discover  that  a  great  part  of  Britain  is  made  of  stone 
like    your  piece  of    sandstone,  for  example  the  hills 
and  dales  of  most    of   Wales,    Lancashire,  and    the 
south  of   Scotland.     And  if  you  climbed  up   to  the 
tops  of  some  of  our  highest  mountains,  such  as  Ben 
Nevis,  you  would  see  them  to  be  built  up  of  solid 
masses  of  granite,  quite  similar  to  your  little  speci- 
men, or  of  other  sorts  of   stones    belonging    to  the 
same  division  as  granite. 

37.  You  would  begin  to  perceive  that  the  different 
kinds  of  stone  are  not  scattered  at  random  over  the 
country,  but  have  each  their  own  places,  with  their 
own  kinds  of  hills  or  valleys. 

38.  But  a  little  further  attention  to  these  matters 
would  bring  before  you  a  far  more  wonderful  thing.    In 


TELL  us.]  GEOLOGY.  17 

questioning  the  stones  about  how  they  were  made, 
you  would  learn  by  degrees  that  each  of  them  can 
give  you  a  more  or  less  distinct  answer.  In  fact  they 
may  be  compared  to  books  each  of  which  has  a  little 
piece  of  history  to  tell. 

39.  You  do  not  grudge  to  read  books  of  history. 
You  find  much  interest  in  following  the  changes  which 
happened  in  old  times  in  your  country,  how  battles 
were  fought  and  laws  were  made,  and   old  customs 
gradually  passed  away.     You  have   no  doubt  found 
that  the  more  you  know  about  these  events  of  former 
times   the  better  do  you    understand  how   the  laws 
and  customs  of   the  present  day  came  to  be  what 
they  are. 

40.  Well,  the  solid  earth  under  your  feet  has  a  history 
as  well  as  the  people  who  have  lived  on  its  surface. 
Take  Britain  for  example.     You  will  learn  that  once 
a  great  part  of  this  country  as  well  as  of  Europe  and 
North  America  was  buried  under  ice  like  Greenland. 
Earlier  still  it  had  jungles  of  palms  and  other  tropical 
plants ;  yet  further  back  it  lay  beneath  a  wide  deep 
ocean;    and  beyond  that  time  can  be   traced  many 
still  more  remote  periods,  when  it  was  forest-covered 

.  land  or  wide  marshy  plains,  or  again  buried  under  the 
great  sea.  Step  by  step  you  may  follow  this  strange 
history  backwards,  and  with  as  much  certainty  as 
you  trace  the  doings  of  Julius  Caesar,  or  William  the 
Conqueror. 

41.  Now  the  records  of  all  these  old  revolutions  of 
the  earth's  surface  are  contained  in  the  stones  beneath 
your  feet.     In  learning  what  these  stones  are,  how 
they  were  made  and  how  they  came  to  be  as  you  now 
see  them,  you  are  really  unravelling  a  part  of  the 


1 8  SCIENCE  PRIMERS.        [SEDIMENTARY 

history  of  the  earth.  Even  the  commonest  bit  of 
stone  has  its  own  part  of  the  story  to  tell  you.  If 
you  are  sure  that  it  was  well  worth  your  while  to  go 
through  the  trouble  of  learning  to  read  for  the  sake 
of  all  the  knowledge  which  you  can  gain  from  books, 
you  will  discover,  too,  that  you  will  be  fully  repaid 
for  any  pains  you  take  in  acquiring  a  knowledge  of 
how  to  read  the  meaning  of  the  stones.  This  earth- 
history  is  written  in  clear  and  legible  language  which 
with  a  little  patience  you  will  easily  master.  And 
when  you  have  once  acquired  it  you  will  not  be  content 
with  what  you  can  learn  from  books.  It  will  then  be 
a  constant  and  increasing  source  of  delight  to  you  to 
get  away  to  the  quarries  and  brooks,  and  sea-shores 
and  hill-sides,  to  any  place  in  short  where  the  rocks 
stick  out  to  the  surface,  that  you  may  question  them 
and  learn  what  they  have  to  tell  about  the  ancient 
revolutions  of  the  earth. 

42.  The  object  of  this  little  book  is  to  set  you  in  the 
way  of  putting  such  questions  to  every  stone  and 
rock  you  may  meet  with.  We  shall  begin  with  the 
very  simplest  lessons  and  appeal  at  every  step  to 
things  which  are  already  familiar  to  you.  In  this  way 
you  will  feel  how  sure  and  steady  your  progress  is,  and 
in  the  end  you  will  be  able  to  carry  on  the  question- 
ing yourselves  without  much  help  from  book  or  friend. 
Hy  watching  what  takes  place  from  day  to  day,  as  in 
a  brook  or  by  the  shore  of  the  sea,  you  will  understand 
the  events  which  have  happened  in  long  past  times, 
and  be  able  to  decipher  among  the  rocks  that  won- 
derful earth-history  which  it  is  the  business  of  Geology 
to  study  and  record. 


ROCKS.]  GEOLOGY.  19 


SEDIMENTARY  ROCKS. 

L  What  Sediment  is. 

43.  We  have  now  advanced  some  way  in  the  attempt 
to  understand  what   stones   are.      We  have  learned 
that  they  are  full  of  a  history  of  old  revolutions  of 
the  earth,  and  that  we  may  find  out  what  this  history 
is,  but  that  in  order  to  make  any  progress  we  must 
arrange  into  distinct  groups  the  various  stones  which 
we  mean  to  study.     We  have  found,  too,  that  they 
may  be  divided  into    three   great  groups  or  classes, 
each  having  a  set  of  well-marked  characters. 

44.  To  each  of  these  groups  names  must  be  given. 
We  might  call  them  the  Sandstone  group,  the  Chalk 
group,  and  the  Granite  group.     But  it  happens  that 
other  names  have  been  already  in  use,  which  will  be 
more    convenient.     Accordingly    we    shall    refer    all 
stones  having  characters  like  those  of  sandstone  to 

•the  Sedimentary  Rocks  ;  those  formed  of  the 
remains  of  plants  or  animals,  as  chalk  is,  to  the 
Organic  Rocks  ;  and  those  having  a  crystalline 
character,  like  our  granite  group,  to  the  Igneous 
Rocks.  The  meaning  of  these  names  will  be  seen 
as  we  proceed. 

45.  The  word  "  rock"  is  applied  to  any  kind  of 
natural  stone,  whatever  may  be  its  hardness  or  soft- 
ness.    In  this  sense,  sand,  mud,  clay,  peat,   and  coal 
are  rocks,  as  much  as  sandstone,  limestone,  or  granite. 

46.  Now  it  is  evident  at  the  very  outset  that  each  of 
these  groups,  since  it  is  so  well  defined  from  the  others, 
must    have   a  history  peculiar    to   itself,    that  is,    its 
various  kinds  of  stone  or  rock  must  have  been  formed 

3 


20  SCIENCE  PRIMERS.        [SEDIMENTARY 

differently  from  those  of  the  other  groups,  in  order  to 
be  so,  unlike  them.  Let  us  then  take  up  each  of  the 
groups  in  succession,  beginning  with  the  Sedimentary 
Rocks,  that  is,  with  those  which  have  a  more  or  less 
close  resemblance  to  sandstone. 

47.  But  first  we  must  understand  the  meaning  of 
this  word,    sedimentary,    and   why  it   is   applied. 
We  take  a  glass  of  water  and  put  some  gravel  into 
it     The  gravel  at  once  sinks  to  the  bottom  and  re- 
mains there  even  though  we  stir   the  water  briskly. 
We  close  the  mouth  of  the  glass  and  shake  it  up  and 
down  so  as  to  mix  the  water  and  gravel  thoroughly 
together ;  but  as  soon  as  we  cease  to  do  so  and  place 
the  glass  on  the  table  again,  we  see  that  the  gravel 
has  sunk  and  formed  a  layer  at  the  bottom.     This 
layer  is  a  sediment  of  gravel. 

48.  Instead  of  gravel  we  put  sand  into  the  water 
and  shake  them  up    as  before.      We  mix  them  so 
completely  that  for  a  moment  or  two  after  we  cease 
the  water  seems  quite  dirty.     But  in  a  few  minutes  the 
sand  will  have  all  sunk  to  the  bottom  as  a  layer  below 
the  water.     This  layer  is  a  sediment  of  sand. 

49.  We  take  a  little  mud  or  clay,  instead  of  either 
the  gravel  or  sand,  and  shake  it  up  in  the  water  until 
the  two  are  thoroughly  mixed.     When  the  glass  is 
replaced  on  the   table  this  time   the  water  continues 
quite  dirty.     Even  after  some  hours  it  remains  still 
discoloured,  but  we  see  a  layer  beginning  to  appear 
at  the  bottom.     If  the  glass  is  allowed  to  stand  long 
enough    undisturbed   that  layer  will   go  on  growing 
until  the  water  has  again  become  clear.     In  this  case 
the  layer  is  a  sediment  of  mud. 

50.  Sediment,  therefore,  is  something  which  after 


ROCKS.]  GEOLOGY. 


having  been  suspended  in,  or  moved  along  by,  water 
has  settled  down  upon  the  bottom.  The  coarser 
and  heavier  the  sediment  the  quicker  will  it  sink, 
while  when  it  is  very  fine  it  may  remain  in  suspen- 
sion in  the  water  for  a  long  time. 

51.  Sedimentary  Rocks  must  thus  be  those  which 
have  been  formed  out  of  sediments.      And  just  as 
sediments   differ  from   each   other  in  coarseness   or 
fineness,  so  will  the  sedimentary  rocks  formed  out  of 
them. 

52.  Here  are  pieces  of  three  sedimentary  rocks  : — 
(i.)  A   piece   of    Conglomerate    or  pudding- 
stone  (Fig.  4). 

(2.)  The  piece  of  Sandstone  you  have  already 

looked  at  (Fig.  i)  ;  and 
(3.)  A  piece  of  Shale  (Fig.  5). 

53.  Examine  the  first  of  these   three   specimens. 
You  find  it  to  be   made    of   rounded    little  stones, 
firmly  cemented  together.     Were  these  round  stones 


FIG.  4. -Piece  of  Conglomerate  or  Pudding-stone. 


to  be  separated  from  each  other,  and  gathered  into  a 
loose  heap,  you  would  call  it  a  heap  of  gravel.  The 
stone  is  evidently  nothing  more  than  a  hardened 


22  SCIENCE  PRIMERS.        [SEDIMENTARY 

gravel,  such  as  you  might  pick  up  on  the  sea-shore, 
or  in  the  channel  of  a  stream.  It  is  sometimes  called 
pudding-stone,  because  the  stones  lie  together  some- 
what like  the  fruit  in  a  plum-pudding. 

54.  Take   up  the  piece  of  sandstone  again,  and 
make  a  further  examination  of  it.     Did  you  ever  see 
anything   like   the    grains  of    which  it  is   made  up? 
You  reply  that  they  are  mere  grains  of  sand  such  as 
might  be  met  with  anywhere.     And  you  are  quite  cor- 
rect.   The  sandstone  consists  of  nothing  else  but  sand 
firmly  held  together  so  as  to  form  a  stone.      If  you 
went  down  to  the  sea-shore,  or  to  the  bed  of  a  brook 
or  river,  you  could  gather  sand  of  very  much  the  same 
kind,  and  by  hardening  such  sand  into  a  compact 
mass,  you  might  make  sandstone. 

55.  In  the  third  specimen  you  cannot  so  easily  make 
out  what  the  grains  of  the  stone  are,  since  their  size 
is  so  small.     But  take  a  knife  and  scrape  a  little  of 


FIG.  5.— Piece  of  Shale. 

the  end  of  the  stone  and  work  it  up  with  some  drops 
of  water.  You  will  make  a  kind  of  paste  in  this 
way.  Then  put  this  paste  into  a  tumbler  of  water 
and  stir  it  well  round.  Immediately  the  water  gets 
dirty-looking,  and  remains  so  even  for  some  time  after- 
wards. But  put  the  tumbler  aside  for  some  hours  and 


ROCKS.]  GEOLOGY.  23 

you  will  find  that  the  water  becomes  clear  again  ;  that 
what  you  put  in  as  a  dirty  paste  has  sunk  to  the 
Bottom  of  the  glass  as  a  layer  of  sediment,  and  that 
it  is  simply  mud.  The  shale,  therefore,  is  nothing 
more  than  a  stone  formed  of  fine  muddy  sediment, 
just  as  the  conglomerate  is  formed  out  of  coarse 
gravelly  sediment. 

56.  Thus  you  see  that  the  term  Sedimentary  Rocks 
is  a  very  expressive  one,  for  it  includes  stones  formed 
of  all  kinds  of  sediment  whether  coarse  or  fine. 

Look  again  at  any  one  of  our  three  specimens,  and 
you  will  understand  that  we  have  two  things  to  find 
out  about  them.  First  of  all,  how  was  the  sediment 
made  out  of  which  they  have  been  formed  ?  and 
secondly,  how  did  the  sediment  come  to  be  gathered 
and  hardened  into  solid  stone  ? 

II.  How  Gravel,  Sand,  and  Mud  are  made. 

57.  You  have  taken  the  first  step  in  the  study  of 
the  Sedimentary  Rocks, — you  now  know  that  they  are 
made  of   sediment   such  as  gravel,  sand,  and  mud. 
The  next  step  must  be  to   find  out  where  this  sedi- 
ment came  from  and  how  it  was  formed.     If  you  can 
settle  this  matter  you  will  evidently  know  a  good  deal 
more  about  the  history  of  these  rocks.     And  here,  as 
in  all  such  matters,  you  will  find  it  well  to  ask  your- 
selves at  the  very  outset : — Is  there  anything  going  on 
now-a-days  which  will  explain  what  we  are  in  search 
of?     By  starting  fresh  from  the  observation  of  what 
takes  place  at  the  present  time,  you  will  be  far  better 
able   to  understand   what  has   been  done  long  ago. 
How  then  are  gravel,  sand,  and  mud  made  at  the 
present  day  ? 


24  SCIENCE  PRIMERS.       [SEDIMENTARY 

58.  A   little   attention  will  show  you  that  the  dif- 
ference between  gravel  and  sand  is  only  one  of  degree 
of  coarseness.     In  gravel  the  stones  are  large,  in  sand 
they  are  mere  grains.     To  make  this  clear,  place  a 
little  sand  under  a  strong  magnifying  glass,  which  will 
make  the  grains  appear  much  larger  than  they  really 
are,  so  large,  indeed,  as  to  give  them  the    look    of 
gravel-stones  rather  than    grains    of  sand.     You  can 
then  see  that  each  grain  is  a  worn,  rounded  stone, 
sometimes    with    little    chips    and    hollows    on    its 
sides,  just  like  those  on  the  sides  of  any  pebble  we 
may  pick  out  of  a  heap  of  gravel.     The  longer  you 
look  at  the  sand  in  this   way  the  more  sure  do  you 
become   that,   after    all,    sand    and   gravel   are    just 
different  states  of  the    same    thing,    the    one    being 
merely  coarser  than  the  other. 

59.  If  you  were  to  search  on  the  shore  of  the  sea, 
or  on  the  banks  of  a  river,  you  could,  without  much 
difficulty,  prove  in  another  way  that  sand  and  gravel 
only  differ  from  each  other  in  the  size  of  their  grains. 
You  might  gather  handfuls  of  fine  sand,  then  of  sand 
a  little  coarser  in  the  grain,  and  so  on  by  degrees 
until  the  material  became  a  true  gravel,  with  rounded 
pieces  of  stone  of  all  sizes,  from  mere  little  pebbles 
up   to  blocks  as  big  as   your  head.      How   did   all 
these  fragments,  whether  small  or  large,  come  to  be 
broken  off  and  ground  so  round  and   smooth,  and 
heaped  up  where  we  now  find  them  ? 

60.  Let  us  get  away  up  among  the  hills,  and  watch 
what  goes  on  where  the  brooks  first  begin  to  flow. 
Where  the  rocks  are  hard  and  tough,  they  rise  out  of 
the   hill-sides   as    prominent   crags   and   cliffs,   down 
which  the  little  streamlets  dance  from  ledge  to  ledge 


GEOLOGY.  25 


before  they  unite  into  larger  streams  in  the  bottoms  of 
the  valleys.  Now  look  at  those  crags.  See  how  they 
are  split  up  and  wasted  by  the  rains  and  frosts.  You 
have  learnt  already  something  about  how  this  is  done 
(Physical  Geography  Primer,  Arts.  126 — 142).  But 
you  have  now  to  consider  some  of  the  results  of  the 
waste. 

6 1.  Suppose,  for  the  sake  of  distinctness,  that  we 
single  out  one  special  crag  where  the  rock  is  of  some 
bright  colour,  say  red,  and  differs  in  that  respect  from 
the  rest  of  the  crags  round   about  it.      It  rises  out 
boldly  from  a  steep  hill- side,  and  looks  down  a  long 
slope  to  the  little  stream  which  in  the  distance  seems  a 
thread  of  silver  winding  through  the  green  meadows 
far  below  us.     Our  crag  has  been  sorely  wasted  in  the 
long  course  of  time.     The  rains  and  frosts  of  many 
centuries  have  carved  its  sides  into  deep  clefts  and 
gullies  (Physical  Geography  Primer,  Art.  142).    These, 
when  wet  weather  sets  in,  become  each  the  channel 
of  a  foaming  torrent,  which  pours  headlong  down  the 
slope  and  sweeps  away  every  loose  bit  of  stone  or 
earth  within  its  reach. 

62.  We  climb  cautiously  along  the  face  of  the  crag 
to  look  into  some  of  these  frost-splintered,  torrent-swept 
gullies,  and  then  we  descend  to  the  base.     All  the  slope 
below  is  strewn  with  pieces  of  the  crag.      Some  of 
these  are  huge  blocks,  but  most  of  the  material  forms 

,a  kind  of  mere  rough  rubbish,  which  slides  down  the 
slope  with  us  as  we  descend  with  long  strides  to  the 
bottom. 

63.  Each  of  the  deep  clefts  which  have  been  scooped 
,  out  of  the  crag  has  a  long  slope  of  this  kind  of  rubbish 

lying  below  it.     You  cannot  for  a  moment  doubt  that 


26  SCIENCE  PRIMERS.        [SEDIMENTARY 

all  this  broken-up  material  on  the  slope  actually 
formed  at  one  time  part  of  the  crag  itself,  that  in  fact 
it  is  simply  the  material  which  has  been  removed  by 
the  slow  wasting  away  of  the  sides  and  bottoms  of 
the  clefts,  and  that  if  you  could  gather  it  all  up  again 
so  as  to  put  it  back  where  it  formerly  stood  you  would 
really  fill  the  clefts  up. 

64.  The  slope  leads  us  down  to  a  little  brook,  the 
bed  of  which  is  strewn  with  pieces  from  our  crag.    Now 
let  us   descend  the  brook  and   look   at   its   channel 
carefully  as  we   go.      The    red    fragments  from  that 
crag  will  be  easily  distinguishable  from  the  other  dull 
grey  stones,  which  have  been  detached  from  the  rest 
of  the  crags  on  either  side.     If  you  look  narrowly  at 
the  bits  of  stone  which  are  strewed  about  upon  the 
slope  you  will  notice  that  they  are  all  more  or  less 
angular  in  shape,  that  is  to  say,  they  have  sharp  edges. 
But  those  in  the  brook  are  not  quite  so  rough  nor  so 
sharp-edged   as  those   on   the   bare   hill-side  above. 
Follow  the  brook  down  the  valley  for  some  way  and 
then  take  another  look  at  the  stones  in  the  bed  of  the 
stream.     You  do  not  now  find  so  many  big  blocks  of 
the  red  stone,  and  those  you  do  meet  with  are  more 
rounded   and   worn  than  they   were   near  the  crag. 
They  have  grown  smooth  and  polished,  their  edges 
have  been   worn   off,    and  many  of  them   are  well 
rounded.     Once  more  you  make  a  further  examina- 
tion still  lower  down  the  valley,  and  here  and  there 
where  the  stream  has  thrown  up  a  bank  of  gravel, 
you  find  that  the  pieces  of  our  red  crag  have  been  so 
well  ground    away  that   they  now  form  part  of    an 
ordinary  water-worn  gravel. 

65.  In  the  same  way  by  descending  the  stream  still 


ROCKS.]  GEOLOGY.  27 

further  you  could  trace  the  gravel  becoming  finer  and 
passing  at  last  into  sand.  And  if  you  were  to  place 
some  of  this  sand  under  a  magnifying  glass  you 
would  find  it  partly  made  up  of  more  or  less  rounded 
grains  of  the  same  red  stone  which  you  detected  in 
the  gravel,  and  which  you  knew  to  have  come  from 
our  crag  far  up  in  the  hills. 

66.  Now  how  is  it  that  the  stones  get  worn  down 
in  this  way  ?     Why  should  lying  in  the  bottom  of  a 
stream  make  them  smaller  ? 

67.  If  you  watch  the  stream  only  in  fine  weather, 
when  the  water  is  low  and  the  current  feeble,  you  can 
hardly  judge  as    to    the    real    power    of  the   water. 
Come  back  when  heavy  rains  have  filled  every  gully 
in  the  hills  with  a  foaming  torrent,  and  when  every 
streamlet  rushes  headlong  down  its  valley,  filling  its 
bed  to  the  brim  and  even  rising  high  on  either  side. 
You  cannot  now  see  the  stones  on  the  bottom  of  the 
channel,  but  listen  and  you  can  hear  them.     That 
sharp  rattle  which  every  now  and  then  comes  out  of 
the  water  is  caused  by  the  stones  thumping  against 
each  other,  as  they  are  hurried  along  by  the  rushing 
water.     They  are  kept  grinding  against  each  other  as 
in  a  mill.     Of  course,   they  must   needs  have  their 
edges  worn  off,  and  their  sides   smoothed,  while  at 
the  same  time  they  smooth  and  polish  the  rocks  of 
the  channel  over  which  they  are  driven. 

68.  When  the  stones  first  fall  or  are  swept  from  the 
hill-side  into  the  brook,  they  are,  as  you  saw,  mere 
angular  chips  (Fig.  6).      But  by  the  time  they  have 
travelled  down  the  brook  a  little  way,  and  have  suffered 
from  the  grinding  of  a  few  floods,  they  lose    their 
sharpness.  The  smoothing  and  polishing  process  goes 


28 


SCIENCE  PRIMERS.        [SEDIMENTARY 


on  till  they  become  more  or  less  rounded,  and  at  last 
appear  as  well-worn  gravel  (Fig.  7).  A  rounded 
stone  will  travel  farther  and  faster  than  an  angular 
one,  but  in  the  end  gets  worn  down  into  mere  sand 
(Fig.  8). 


FIG.  6. — Stones  detached  from  a  cliff"  by-  rains,  frosts,  &c.,  and  launched  into 
a  brook. 

69.  Thus  we  see  that  as  the  stones  grow  rounder 
they  at  the  same  time  become  smaller.  And  not  only 
do  they  wear  away  each  other,  they  also  grind  out 
the  sides  and  bottom  of  the  channel  of  the  brook. 


FIG.  7. — Stones  from  same  cliff  after  having  been  rolled  about  in  the  bed  of 
the  brook. 

A  good  deal  of  stone  must  be  consequently  rubbed 
down  (see  Physical  Geography  Primer,  Art.  175).  Now 
it  is  this  worn  material  which  makes  gravel,  sand, 
and  mud.  In  the  bed  of  every  stream  you  will  never 
fail  to  find  plenty  of  this  worn  material,  derived  from 
the  rubbing  away  of  stones  by  water. 


ROCKS.] 


GEOLOGY. 


29 


70.  The  finer  particles,  being  more  easily  moved, 
travel  much  farther  than  the  coarser  fragments. 
Hence,  while  the  gravel  and  coarse  sand  are  pushed 


PIG.   8. — A  small  heap  of  sand  consisting  of  pieces  of  stone  from  the  same 
cliff  which  have  been  still  further  worn  away  in  the  bed  of  the  brook. 

along  the  bottom,  the  fine  sand  and  mud  are  sus- 
pended in  the  moving  water  and  may  be  carried  by 
it  for  many  miles  before  they  slowly  sink  to  the 


FIG.  9. — A  glass  of  water  taken  from  the  same  brook  when  in  flood,  to  show 
how  the  finer  particles  worn  from  the  same  stones  settle  down  on  the 
bottom  as  a  layer  of  mud. 

bottom    to    form   there   a    deposit   of    silt   or    clay 
(Fig.  9)- 

71.  You  will  see  from  this,  that  while  the  brooks  in 


30  SCIENCE  PRIMERS.        [SEDIMENTARY 

the  higher  parts  of  a  country  may  have  their  channels 
encumbered  with  big  blocks  of  rock,  and  quantities  of 
coarse,  sharp,  angular  rock-rubbish,  all  this  material 
is  worn  down  by  degrees,  and  reaches  the  lowlands 
or  the  sea  only  as  fine  sand  and  mud.  As  the 
brooks  are  always  flowing,  so  are  they  always  trans- 
porting the  worn  materials  of  the  hills.  But  as  fast 
as  they  do  so,  the  hills  are  crumbling  down  and  sup- 
plying fresh  materials  to  the  brooks.  So  that  the 
amount  of  gravel  and  sand  ground  up  every  year 
even  by  the  comparatively  small  streams  of  this 
country  must  be  enormously  great.  (See  Physical 
Geography  Primer,  Arts.  170 — 182.) 

72.  We  can  now  return  to  our  crag  of  red  rock  with 
freshened  interest.      Every  cleft  and  gully  which  has 
been  worn  into  its  sides,  bears  witness  to  the  general 
crumbling  which  the  surface  of  the  land  undergoes. 
We  may  follow  its  ruined  blocks  and  rubbish  into  the 
brook  below,  watch  how  they  are  ground  down  there, 
and  trace  them  onwards  until  in  the  form  of  fine  silt 
and  mud  their   remains  find  their  way  at  last  into 
the  far  distant  plains  and  thence  to  the  bottom    of 
the  great  sea. 

73.  But  it  is  not  only  in  the  beds  of  brooks  and 
rivers  that  you  can  watch  how  the  hardest  rocks  are 
ground  away  into  gravel  and  sand.    Look  at  any  of 
the  rocky  parts  of  the  coast  line  of  this  country  and 
there  mark  the  effects  of  the  waves  of  the  sea.     If 
a   cliff  rises   from   the   upper    edge    of    the   beach, 
you  can  at  once  tell  which  parts  are  exposed  to,  and 
which   lie   beyond  the  reach  of  the  waves.      Over- 
head the  cliff  is  rough  and  splintered  where  merely 
rain,  frost,  or  springs  have  acted  on  it  (see  Physical 


ROCKS.]  GEOLOGY.  31 

Geography  Primer,  Arts.  137,  138).  But  towards  its 
base  the  rocks  have  been  ground  smooth  and  polished 
like  those  in  the  bed  of  a  mountain-brook.-  What 
has  smoothed  the  bottom  of  the  cliff  and'  left 
all  the  higher  parts  rough  and  crumbling  ?  The 
waves  have  done  it. 

74.  Huge  slices  of  the  weather-roughened  cliff  have 
been  detached  and    have   fallen  down  on  the  beach 
below.     Others  are  ready  to  tumble  off.    Examine  the 
fallen  blocks  and  you  will  see  that  usually  only  those 
lying  at  the  base  of  the  cliff,  and  which  have  not  yet 
been  moved  by  the  waves,  have  still  their  sharp  edges. 
A  little  lower  down  the  blocks  show  signs  of  having 
been  ground  'together,  while  the  greater  part  of  the 
beach  is  strewn  with  stones  of  all  sizes,  well  rounded 
and  polished. 

75.  On  a  calm  day  when  only  little  wavelets  curl  on 
the  shore  you  cannot  easily  judge  what  the  sea  really 
does  in  the  way  of  grinding  down  the  beach  and  the 
bottom  of  the  cliffs,  just  as  you  could   not  form  a 
proper   notion   of  the  work  of  a  brook   merely  by 
seeing 'it  lazily  creeping  along  its  bed  in  a  season  of 
drought.     But  place  yourselves  near  a  cliff  during  a 
storm,  and  you  will  not  need  any  further  explanation 
as  to  the  power  of   the  waves   to  grind  down  even 
the  hardest  rocks.     Each  huge  breaker  as  it  comes 
tossing  and  foaming  up  the  beach  lifts-  up  the  stones 
lying  there  and  dashes  them  against  the  base  of  the 
cliff  where  it  bursts  into  spray.     As  the  green  seething 
water  rushes   back  again  to  make  way  for    the  next 
wave,  you  can  hear,  even    perhaps   miles   away,  the 
harsh  roar  of  the  gravel  as  the  stones  grate  and  grind 
on    each    other  while    they  are    dragged    down    the 

4 


32  SCIENCE  PRIMERS.         [SEDIMENTARY 

beach,  only  to  be  anew  caught  up  and  swept  once 
more  towards  the  base  of  the  cliff.  You  could  not 
conceive  of  a  more  powerful  mill  for  pounding  down 
rocks  and  converting  their  fragments  into  well-worn 
gravel  and  sand  (Physical  Geography  Primer,  Arts. 
230 — 232).  Just  as  in  the  channel  of  every  stream 
so  along  the  shores  of  every  sea  you  meet  with  the 
fragments  of  the  rocks  of  the  land  in  all  stages  of 
destruction,  from  the  big  angular  block  down  to  the 
finest  sand  and  mud. 

76.  If,  therefore,  I  now  repeat  the  question,  "How 
are  Sand  and  Gravel  made  ?  "  you  will  at  once  answer 
— "Sand  and  Gravel  are  part  of  the  material  worn 
away  from  the  surface  of  the  land,  and  ground  down 
in  moving  water."     Materials  which  have  been  rubbed 
smooth   in  this  way  are  said   to   be    "  water-worn." 
But  you  will  now  see  that  it  is  not  the  water  which  of 
itself  wears  them  away.     They  are  in  fact  worn  away 
by  themselves,  and  all  that  the  water  does  is  to  keep 
them  moving  and  grinding  against  each  other. 

III.  How  Gravel,  Sand,  and  Mud  become 
Sedimentary  Rocks. 

77.  We  have  now  got  so  far  on  our  way  as  to  under- 
stand  whence    the   materials  of   which   sedimentary 
rocks  are  made  were  derived.     But  the  further  ques- 
tion remains,  how  have  these  materials  been  gathered 
together  and  hardened  into  solid  stone  ?     As  before, 
we  must  find  the  answer  to  such  questions  in  what 
we  can  see  going  on  around  us.      By  turning  back 
again  to  the  brooks,  rivers,  and  sea,  we  shall  get  this 
next  matter  very  clearly  explained. 


ROCKS.]  GEOLOGY.  33 

78.  Water  flows  more  quickly  down  a  steep  slope 
than  over  a  gentle  one.      You  know  that  when  you 
raise  one  end  of  a  tray,  water  poured  on  it  runs  down 
to  the  lower  end,  and  does  so  the  faster  the  steeper 

'you  make  the  inclination. 

79.  If  you  put  crumbs  or  pebbles  of  different  sizes 
on  the  tray  you  will  notice  that  they  a;e  swept  down 
more  by  the  rapid  than  by  the  slower  flow  of  water. 
A  quickly  flowing  current  of  water  is  more  powerful 
to    move    anything    than    one    which    flows    slowly. 
Hence,  as  you  will  at  once  see,  there  must  be  great 
differences  in  the  size  and  weight  of  materials  which 
different  streams  or  different  parts  of  the  same  stream 
can  move. 

80.  So  long  as  a  current  of  water  is  moving  swiftly 
it  keeps  the  gravel,  sand,  and  mud  from  settling  down 
on  the  bottom.      You  remember  that  when  you  put 
some  of   each  of   these  materials  into  glasses,  and 
kept  the  water  in  rapid  motion,  they  continued  sus- 
pended in  the  water,  and  only  sank  to  the  bottom  as 
the  water  began  to  lose  its  motion,  the  gravel  first, 
then  the  sand,  and  last  of  all  the  mud  (Arts.  47 — 49). 
Now  this  is  just  what  takes  place  in  all  the  moving 
waters   of  the   globe.      A   rapid    current   will   hurry 
along,  not  only  mud  and  sand,  but  even  gravel.     As 
its   rapidity  flags,  first   the   gravel   will   sink   to   the 
bottom  as  a  sediment,  the  sand  will  sink  more  slowly 
and  be  carried  further,  while  the  mud  will  hang  in  the 
water  for  a  long  time,  travel  a  much  greater  distance, 
and  only  fall  with  extreme  slowness  to  the  bottom. 

8 1.  You  must  test  the  truth  of  these  statements  the 
first  time  you  have  an  opportunity  of  looking  into  the 
rocky  channel  of  a  brook  as  it  escapes  from  the  hills. 


34  SCIENCE  PRIMERS.         [SEDIMENTARY 

Get  to  some  part  where  the  water,  shooting  swiftly 
over  ledges  and  rocks,  has  strength  enough  to  sweep 
even  big  blocks  of  stone  along  with  it.  A  little  way 
further  down  you  will  find  the  channel  less  steep 
and  the  current  less  strong.  Now  look  into  the 
bottom  of  the  stream.  Is  it  covered  with  fine  mud  ? 
Assuredly  not.  You  meet  only  with  big  blocks  of 
stone  and  coarse  gravel.  These  have  been  dropped 
as  soon  as  the  water  had  its  force  checked  by  coming 
from  a  steep  to  a  more  level  part  of  its  course. 
But  it  still  had  power  enough  to  transport  the  finer 
sorts  of  sediment.  You  need  to  go  further  down 
towards  the  low  grounds  before  you  see  the  bed  of 
the  stream  covered  with  sand,  and  much  further  yet, 
even  far  into  the  plains,  before  you  meet  with  layers 
of  mud. 

82.  After  seeing  these  things  with  your  own   eyes, 
you  would  be  convinced  that  wherever  you  find  masses 
of  gravel  they  tell  you  of   strong  currents  of   water, 
that  beds  of  sand  point  to  less  rapid  currents,  while 
sheets  of  mud  show  where  the  water  has  had  either  a 
very  gentle  motion  or  has  been  quite  still  so  as  to  let 
the  fine  sediment  settle  down  quietly  on  the  bottom. 

83.  Now  see  how  important  this  knowledge  becomes 
when  you  begin  to  inquire  how  different  stones  were 
made.     If  you  have  ascertained  clearly  how  various 
kinds  of  sediment  are  formed,  you  have  got  a  long 
way  towards  understanding  how   Sedimentary  Rocks 
came  to  be  made.     These  rocks  may  be  hard  stone 
now,  and  may  be  used  for  paving  streets  or  building 
houses.     But  you  have  learnt  that  mere  hardness  or 
softness  goes  for  little,  and  that  it  is  the  materials  of 
which  the  stone  consists  that  you  have  to  consider. 


ROCKS.]  GEOLOGY.  35 

When  you  find  these  materials  to  be  water-worn 
grains  of  mud,  sand,  or  gravel,  you  confidently  assert 
that  no  matter  how  hard  the  stone  may  be  now, 
it  was  once  in  the  state  of  mere  loose  sediment  under 
water. 

84.  But  you  can  tell  more  than  this.     By  seeing  the 
kind  of  sediment  of  which  a  rock  is  made  up  you 
know  something  about  the  nature  of  the  water  in 
which   the  materials    of   the  rock    were   laid    down. 
For  instance,  you  recognize  a  rock  of  conglomerate 
to    be    only   a    mass  of  compact   gravel,    and    you 
are    sure  that  like  ordinary  gravel  now-a-days  it  was 
rolled  about  in  shallow  water  such  as  the  bed  of  a 
lake  or  river  or  on  the  shore  of  the  sea.     Again,  you 
see  in  a  rock  formed  of  fine  mud,  such  as  shale,  proofs 
of  deeper  or  stiller  water  into  which  only  the  finer 
particles  worn  away  from  the  land  were  carried. 

85.  We  have  watched  how  the  sediments  are  ground 
down  by  brooks,  rivers,  and  waves ;  let  us  now  follow 
them  until  they  are  gathered  into  places  where  they 
can  accumulate  without  being  constantly  washed  away. 

86.  Some  account  has  already  been  given  (Physical 
Geography  Primer,  Arts.   147  and  182)  of  what  be- 
comes of  the  materials  worn  away  from  the  surface  of 
the  land.      You  have  learned  how  they  are  washed 
down  by  rains  into  brooks  and  rivers,  how  they  are 
there  ground  down,  and  how  finally  they  are  borne  as 
fine  sand  and  mud  away  out  to  the  bottom  of  the  sea. 

87.  Now  these  deposits  of  sediment  over  the  sea 
bottom  will  by-and-by  become  hard  sheets  of  stone, 
like  the  common   sedimentary  rocks  we  have  been 
dealing  with  in  these  lessons.     You  cannot  see  what 
goes  on  under  the  sea,  but  you  can  form  some  notion 


36  SCIENCE  PRIMERS.         [SEDIMENTARY 

of  it  by  watching  what  takes  place  in  pools  of  water 
on  the  land. 

88.  Let  us  suppose  that  we  know  a  muddy  street  or 
road  which  slopes  down  gently  to  a  more  level 
part,  and  that  in  wet  weather  the  rain  gathers  in 
pools  at  the  bottom  of  the  slope.  We  choose  a  wet 
day,  and  after  following  the  course  of  one  of  the 
gutters  down  the  slope  and  noticing  how  the  muddy 
water  sweeps  along  sand,  gravel,  bits  of  cork,  stick, 
paper,  and  whatever  lies  in  its  way,  we  halt  at  a 
large  pool  which  has  gathered  on  the  road,  and  into 
which  the  current  of  muddy  water  is  discharging 
itself.  So  long  as  the  water  flows  quickly  downward 
it  sweeps  away  gravel  and  sand.  But  see  what 
happens  when  it  begins  to  flow  more  slowly  over  the 
flat  at  the  bottom  and  enters  the  pool.  By  losing 
speed  it  loses  carrying  power,  and  must  needs  drop 
some  of  its  burden  of  sediment.  The  heaviest  par- 
ticles fall  to  the  bottom  first,  and  tL'is  takes  place  just 
where  the  current  is  checked  by  meeting  the  level 
water  of  the  pool.  Now  mark  the  result.  That  part 
of  the  pool  where  the  current  enters  is  gradually  filled 
up,  except  the  channel  which  the  current  keeps  open 
for  itself.  You  can  see  how  this  tongue  of  sediment 
is  advancing  upon  the  water,  and  that  it  will  in  the 
end,  should  the  rain  last  long  enough,  fill  the  pool 
up  entirely.  It  is  only  the  coarse  sand  which  col- 
lects there;  the  fine  mud  goes  across  the  pool,  and 
though  part  of  it,  as  you  will  find,  settles  down  on  the 
bottom,  much  or  most  of  it  escapes  at  the  further  end 
of  the  pool,  because  the  water  has  not  had  time,  in 
its  passage  from  the  one  side  to  the  other,  to  drop 
its  burden  of  mud. 


ROCKS.]  GEOLOGY.  37 

89.  Let  us  suppose  further  that  when  the  rain  has 
ceased,  no    cart-wheel   or  other  intruder   comes   to 
disturb  our  pool,  but  that  the  water  is  suffered  quietly 
to    soak   into  the  ground  and  to  evaporate,  so  that 
in  a    day    or    two    the    hollow    is    laid   dry.       You 
can  now  examine  the   bottom  of    the  pool  and  see 
exactly    what   took   place   when    the    muddy    water 
filled  it.     Here  at  the  upper  end  is  the  tongue  of 
sand   pushed   out  from  the   shore  by  the  streamlet. 
You    recognize    it   as  a   true    Delta    (Physical  Geo- 
graphy Primer,  Art.   181).     The  bottom  of  the  rest  o'f 
the   pool    is   covered  with  fine  muddy  silt   or  sand 
spread  out  over  all  the  space  on  which  the  water  lay. 

90.  With  a  knife  we  carefully  cut  a  hole  or  trench 
through  these  deposits  on  the  floor  so  as  to  learn  what 
they  consist  of  from  top  to  bottom.      A  cutting  of 
this  kind  is  called  a  Section,   and  may  be  of  any 
size.     The  steep  side  of  a  brook,  the  wall  of  a  ravine, 
the  side  of  a  quarry  or  railway-cutting,  a  line  of  cliff, 
are   all  sections  of  the  rocks.     Let  us  see  what  our 
section  has  to  tell. 

91.  In  the  centre  of  the  little  basin  the  sediment 
brought  in  by  the  rain  has  accumulated  to  a  depth, 
let  us  say,  of  an  inch,  below  which  lies  the  ordinary 
surface  of  the  roadway.     Now  what  feature  strikes  you 
first  about  this  deposit  of  sediment  when  you  come 
to  look  at  the  section  which  we  have  cut  through  it  ? 
Are  the  materials  arranged  without  any  order?    By  no 
means.       If  you  made  a  drawing  of  the   section  it 
might  be  something  like  the  following  woodcut  (Fig, 
10).     The   materials  have  been  deposited  in  layers 
which  have  been  laid  down  flat  one  above  another. 
Some  of  these  layers  are   finer,   others  coarser  than 

'M  i 


38  SCIENCE  PRIMERS.         [SEDIMENTARY 

the  rest.     But  whether  coarse  or  fine  they  all  show 
the  same  general  arrangement  in  level  lines. 

92.  In  looking  at  these  layers  you  can  follow  exactly 
how  each  of  them  was  deposited.  The  coarse  sedi- 
ment is  seen  chiefly  at  the  bottom,  and  marks  where 
the  stronger  currents  carried  sand  and  bits  of  stone 
across  the  pool.  But  as  the  rain  slackened,  the 
runnels  on  the  roadway  grew  less  and  the  currents  in 
the  pool  became  feebler.  Hence,  instead  of  coarse 
sand,  only  fine  silt  was  deposited,  so  that  in  the 


>  v  :<  c^   a 

FIG.  io.  — Section  or  cutting  through  the  sediment  brought  by  rain  into  a 
pool  on  a  roadway,  a.  Surface  of  roadway,  b.  Layers  of  coarse  sand 
with  bits  of  coal  and  ash.  c.  Layer  containing  twigs,  bits  of  straw,  leaves, 
paper,  &c. 

upper  half,  the  layers  are  finer  than  they  are  in  the 
lower.  Together  with  the  sand,  gravel  and  mud,  you 
may  notice  chips  of  wood,  leaves  and  twigs  (c  in  Fig. 
TO)  which  have  been  laid  down  among  the  layers  of 
sediment. 

93.  You  may  think  perhaps  that  observations  such 
as  these  are  too  trifling,  and  that  surely  it  cannot  matter 
what  rain  may  do  in  a  little  pool  on  a  roadway,  since 
we  are  not  to  judge  of  the  world  at  large  by  what 
goes  on  upon  so  small  a  scale.  In  reality,  however, 


ROCKS.]  GEOLOGY.  39 

if  you  thoroughly  understand  what  takes  place  over 
the  bottom  of  such  a  pool,  insignificant  though  it  may 
seem,  you  lay  a  foundation  from  which  it  will  be  easy 
for  you  to  understand  how  sedimentary  rocks  are  and 
have  been  formed  all  over  the  world. 

94.  Instead  of  the  pool  imagine  to  yourselves  a  great 
lake  such  as  that  of  Geneva,  and  in  place  of  the  mere 
tiny  runnel  on  the  road,  formed  by  the  sudden  rain, 
and   disappearing   when    the   rain    ceases,  picture   a 
great  river  like  the  Rhone,  ever  fed  by  the  rains  and 
snows  and  springs  of  a  huge  mountain  chain.     And 
yet  though  you  make  the  scale  on  which  the  work 
goes  on  greater,  the  kind  of  work  remains  the  same 
as  in  the  pool.      You  look  with  wonderment  on  the 
river  rushing  so  swiftly  past,  and  tossing  its  muddy 
waters    into   wave   and   foam,    from   bank   to   bank. 
You  watch   it    enter   the    lake,  and  mark   how   the 
waves  one  by  one  sink  down,  and  how  the  river  loses 
itself  and  its  tumult  in  the  quiet  silent  water  of  the 
deep  blue  lake. 

95.  But   climb   one  of  the   mountains  which   rise 
steeply  from  either  side  of  the  upper  end  of  the  Lake 
of  Geneva.    When  you  get  up  a  few  hundred  feet,  turn 
and  look  down  upon  the  river  and',  lake,  and  see  if 
they  do  not  strongly  remind  you  of  our  runnel  and 
pool  on  the  road.      The  bottom  of    the  valley  lies 
spread  out  as  in  a  map  before  you.     The  windings  of 
the  river,  the  flat  green  meadows  on   either  side  run- 
ning as  a  long  tongue  into  the  lake,  the  little  cottages 
and  hamlets,  and  the  lines  of  road — all  so  dwindled 
down  in  the  distance  that  you  can  see  at  a  glance  how 
they  lie.     That  green  tongue  of  meadows  filling  up 
the  upper  end  of  the  lake  and  stealing  along  each  side 


40  SCIENCE  PRIMERS.         [SEDIMENTARY 

of  the  river  is  the  Delta.  It  has  been  formed  in  the 
same  kind  of  way  as  the  little  delta  in  our  pool,  only 
instead  of  hours  it  has  needed  thousands  of  years  for 
its  formation.  About  a  mile  and  a  half  from  the  edge 
of  the  lake,  a  little  hamlet,  standing  among  the  level 
fields,  was  actually  at  the  margin  of  the  water  some 
eighteen  hundred  years  ago,  and  is  still  called  Port 
Vallais.  The  river  has  thus  pushed  out  its  delta  and 
filled  up  the  lake  for  a  mile  and  a  half  since  Roman 
times. 

96.  From  the  high  ground  overlooking  the  head  of 
the  lake  you  can  see  moreover  another  curious  fact 
about  the  way  in  which   the  sediment  gathers  over 
the  bottom.     The  Rhone  is  very  muddy,  and  as  the 
mud  has  a  white  colour  here,  the  milky  look  which 
it  gives  to  the  water  enables  you  to  follow  the  course 
of  the  river  into  the  clear  blue  lake.     Looking  down 
upon  it  from  the  heights  you  can  trace  the  pale  muddy 
current  for   some   way  out  from    the  shore  until    it 
gradually  gets  mixed  with  the  lake-water  and  disap- 
pears. 

97.  Go  now  to  the  lower  end  of  the  lake,  and  watch 
where  the  water  escapes.     Do  you  see  any  mud  now? 
No,    your   eyes   never   looked    on   clearer,    brighter, 
bluer,    water   than   that    which   comes   rushing    and 
leaping  between  the  banks  and  beneath  the  bridges 
of   Geneva.     What  has  come  of  that  cloud  of  pale 
mud   which   you  saw  carried  by  the   river  into    the 
upper  end  ?    It  has  all  settled  down  upon  the  bottom. 
Day  by  day,  year  by  year,  and  century  after  century, 
the  cloud  of   mud   is  there,   always   sinking  through 
the  water  to  the  bottom,  and  always  renewed  by  the 
restless  river. 


GEOLOGY.  41 


98.  Could  you  drain  off  all  the  water  of  the  lake  you 
would  find  the  floor  covered  with  deposits  of  sedi- 
ment stretching,  not  over  a  few  square  feet,  as  in  our 
little  wayside  pool,  but  over  many  square  miles.     The 
coarser   sediments-»-shing]es   and   gravels — would  be 
met  with  at  the  upper  end  where  the  strong  current 
flowed,  while  the  finer  sediments — sand  and  mud — 
would  cover  the  main  part  of  the  bottom. 

99.  If  you  were  to  bore  through  these  deposits,  you 
would  find  them  in  some  places  to  be  perhaps  more 
than  a  hundred  feet  thick,  and  digging  down  anywhere 
among  them  you  would   see  the  same  arrangement 
into  flat  layers  which  you  observed  in  the  rain-pool. 
Sand,  mud,  and  gravel  might  follow  each  other  from 
top  to  bottom,  but  always  in  beds  or  layers  lying  one 
above  another. 

100.  The  Lake  of  Geneva  is  many  thousand  times 
larger  than  our  little  pool ;  and  yet  it  is  itself  only  a 
pool,  and  a  very  small  one,  when  compared  with  the 
great  sea.     Go  to  the  margin  of  the  sea  where  a  large 
river  enters,  and  you  will  see  that  mere  size  does  not 
alter  the  kind  of  work  which  the  river  and  the  sea  are 
doing,  and  that  in  their  case  too  you  have  the  same 
process   to   study  which  you  have  watched  already. 
You  perceive  how  the   river  is  continually  carrying 
vast  quantities  of  sand  and  mud  into  the  sea.     You 
can  follow  the  muddy  river-water  to  a  distance  from 
the  shore  until,  as  its  mud  slowly  sinks  to  the  bottom, 
it  loses  itself  in  the  waters  of  the  ocean.     You  know 
that  by  this  means  the  bottom  of  the  sea  for  a  long 
way  from  the  coast  is  constantly  receiving  fresh  de- 
posits of  sand  and  mud  which  have  been  washed  off 
the  land.     The  upper  edge  of  these  deposits  is  un- 


42  SCIENCE  PRIMERS.          [SEDIMENTARY 

covered  when  the  tide  goes  out.  You  can  dig  into 
them  where  they  form  the  beach,  and  when  you  do 
so  you  recognize  the  same  arrangement  into  layers 
as  you  found  to  be  the  case  elsewhere. 

IQI.  In  this  way  you  gradually  would  come  to  be 
convinced  that  one  grand  leading  feature  of  the  sedi- 
mentary deposits  laid  down  under  water  is  that  they 


FIG.  ii.— Stratification  of  Sedimentary  Rocks,    a.  Conglomerate      b.  Sand- 
sione.     c.  Shale. 

are  not  mere  random  heaps  of  rubbish,  but  that  they 
are  assorted  and  spread  over  each  other  in  regular 
layers.  This  kind  of  arrangement  is  called  Strati- 
fication, and  the  sediments  so  arranged  are  said  to 
be  stratified.  So  characteristic  is  this  mode  of 
arrangement  among  the  sedimentary  rocks  that  they 
are  often  called  also  the  Stratified  Rocks. 

102.  The  sheets  of  sand,  gravel,  or  mud  which 
can  be  seen  on  the  sea-shore,  or  at  any  lake  or 
pool  on  land,  are  soft  or  loose  materials.  Sand- 
stone, conglomerate,  shale,  or  any  other  sedimentary 
rock,  is  usually  more  or  less  hard  or  compact. 


ROCKS.]  GEOLOGY. 


How  is  this  difference  to  be  accounted  for?  You 
are  quite  sure  that,  in  spite  of  their  firmness,  these 
rocks  were  once  mere  loose  sediment  formed  under 
water  in  the  same  way  as  sediment  is  made  every- 
where at  the  present  day.  But  what  has  turned 
them  into  hard  stone  ? 

103.  If  you  take  a  quantity  of  mud,  and  place  it 
under  a  weight  which  will  squeeze  the  water  out  of  it, 
you  will  find  that  it  gets  firmer.    You  can  thus  harden 
it  by  pressure.      Again,  if  you   place  some   sand 
under   water  which    has   been    saturated   with   lime 
(that  is,  the  material  of  which  chalk  and  limestone 
are  made)  or  with  iron,  or  with  some  other  mineral 
which  can  be  dissolved  in  water,  you  will  notice  that 
as  the  water  slowly  evaporates  it  deposits  its  dissolved 
material  round  the  grains  of  sand  and  binds  them 
together.      Were   you  to  continue  this  process  long 
enough,  adding  more  of  the  same  kind  of  water  as 
evaporation  went  on,  you  would  convert  the  loose 
sand  into  a  solid  stone.     In  this  case  the  hardening 
of  the   sediment  into  stone  would  be    done  by  the 
process  called  infiltration. 

104.  In  one  or  other  or  both  of  these  ways  most  of 
the  sedimentary  rocks  have  been  hardened  into  the 
state  in  which  we  now  find  them.    AVhen  sand  and  mud 
are  piled  up  over  each  other  in  wide  sheets  or  layers, 
to  a  depth  of  hundreds  or   thousands  of  feet,  the 
layers  at  the  bottom,  lying  under  such  an  enormous 
weight,  must  be  squeezed  into   a  much  firmer  con- 
dition than  those  at  the  top.     But  besides  this,  water  is 
always  filtering  through  pores  and  cracks  of  the  rocks, 
sometimes  removing,  sometimes   depositing,   mineral 
matter  (in  the  way  explained  in  the  Physical  Geo- 


44  SCIENCE  PRIMERS.         [SEDIMENTARY 

graphy  Primer,  Arts.  1 1 7 — 125),  and  helping  to  cement 
the  grains  of  many  rocks  more  firmly  to  each 
other. 

105.  If  I  were  now  to  ask  you  what  an  ordinary 
sedimentary  rock  is,  you  would  readily  give  me,  and 
clearly   understand,    such   a    definition  as    this — "A 
sedimentary  rock  is  one  formed  from  sediment  which 
was  derived  from  the  waste  of  older  rocks,  and  de- 
posited  in   water.       It   usually  shows   the    stratified 
arrangement  characteristic  of  water-formed   deposits. 
Since  its  original  formation  it   has  usually  been  har- 
dened into  stone  by  pressure  or  infiltration." 

IV.  How  the  Remains  of  Plants  and 
Animals  come  to  be  found  in  Sedimen- 
tary Rocks. 

1 06.  Although  sedimentary  rocks  consist  of  such 
materials  as  gravel,  sand,  or  mud,  they  often  contain 
other  things  quite  as  interesting  and  important.     For 
example,  here   are   two  additional  pieces   of   Shale 
(Figs.  12  and  13),  in  which  you  will  see  certain  objects 
very  different  from  the    ordinary  sediment  of  which 
the  stone  is  made.     Let  us  first  satisfy  ourselves  as  to 
what  these  objects  are,  and  then  as  to  how  they  came 
to  be  imbedded  in  the  stone. 

107.  We  begin  with  the  specimen  which  is  drawn 
in  Fig.   12.     In  the  stone  itself  you  would  recognize 
merely  a  fragment  of  common  shale,  formed  of  the 
same  materials,  and  arranged  in  the  same  stratified 
way  as  in  your  former  specimen  of  that  rock. 

108.  But  what  is  this  black  object  lying  on  the  upper 
surface  of  the  stone  ?     You  see  at  once  that  it  has  the 


ROCKS.]  GEOLOGY.  45 

form  of  a  plant  and  resembles  some  of  the  fern  tribe. 
Examine  it  more  closely,  and,  tracing  the  delicate 
veil  ling  of  the  fronds,  you  cannot  doubt  that,  although 
no  longer  soft  and  green,  it  was  once  a  living  fern. 
It  has  been  changed  into  a  black  substance  which, 


FIG.  12.— Piece  of  Shale  containing  portion  of  a  fossil  fern. 

when  you  look  carefully  at  it,  proves  to  be  a  kind  of 
coal.  Little  fragments  and  layers  of  the  same  black 
coaly  substance  may  occur  throughout  the  piece  of 
shale.  If  you  scrape  a  little  off  and  put  it  upon  the 
point  of  a  knife,  you  find  you  can  burn  away  the 
black  material  while  the  grains  of  sand  or  clay 
remain  behind.  These  fragments  and  layers  are 
evidently  only  leaves  and  bits  of  different  plants  im- 
bedded at  the  same  time  as  the  larger  and  better 
preserved  fern.  Now  how  did  plants  find  their  way 
into  the  heart  of  a  piece  of  stone  ? 

109.  To  understand  how  this  happened  we  must 


46  SCIENCE  PRIMERS.          [SEDIMENTARY 

again  go  back  to  what  nature  is  doing  at  the  present 
time.  You  remember  that  when  you  were  watching  the 
runnel  coursing  down  the  sloping  roadway  (Art.  88), 
you  noticed  that  it  sometimes  swept  along  bits  of  straw, 
wood,  paper,  or  other  loose  objects  which  it  managed 
to  reach.  Some  of  these  floated  away  into  the  nearest 
drain  and  were  soon  lost  sight  of.  But  others  sank  to 
the  bottom  of  our  little  pool.  Look  again  at  the 
section  we  cut  open  there  (Fig.  10),  and  you  will  find 
little  chips  of  wood  or  straw  or  leaves  and  blades  of 
grass  among  the  fine  sand  and  mud  left  by  the  rain. 
These  objects  lie  flat  between  the  thin  layers  of  sedi- 
ment. And  if  you  think  of  it  you  will  understand 
how  that  should  be  the  position  they  would  naturally 
take  as  they  sank  to  the  bottom.  Rain  therefore  can 
wash  away  leaves  and  other  pieces  of  plants,  and  allow 
them  to  drop  in  a  pool,  where  they  become  inter- 
stratified  with  the  silt,  that  is,  are  deposited  between 
its  layers  and  covered  over  by  it. 

1 10.  Again  :  watch  what  takes  place  along  the  banks 
or  at  the  mouth  of  a  river,  and  you  will  soon  observe 
that  the  leaves,  branches,  and  other  floating  objects 
carried  down  by  the  current  in  the  end  sink  to 
the  bottom,  there  to  be  imbedded  in  and  gradually 
covered  up  by  the  growing  accumulation  of  sand  and 
mud.  If  you  dig  into  any  of  the  deposits  along  the 
banks  you  meet  sometimes  with  layers  of  leaves  and 
twigs,  grouped  in  the  same  stratified  way  as  the 
sediment  above  and  below  them.  Such  deposits  of 
drifted  vegetation  often  form  a  conspicuous  part 
of  the  accumulations  of  which  the  delta  of  a  river 
consists.  (Physical  Geography  Primer,  Art.  180.) 

in.  But  it  must  happen  continually  that  before  the 


ROCKS.]  GEOLOGY.  47 

leaves  or  branches  or  the  trunks  of  trees  have  become 
so  saturated  or  water-logged  as  to  sink  to  the  bottom 
they  are  borne  onward  into  the  sea.  In  such  cases 
they  may  float  a  long  way  from  shore  ere  they  fall  to 
the  bottom  and  become  buried  in  the  silt  and  sand 
there.  Hence,  whether  in  the  beds  of  rivers,  or  at  the 
bottom  of  lakes  or  of  the  sea,  the  remains  of  land 
plants  must  be  constantly  dropping  among  the  sedi- 
mentary deposits  which  are  gathering  there. 

112.  You  can  now  see  therefore  how  it  is  that  pieces 
of  ferns  or  any  other  kind  of  land  plants  should  be 
found  in  the  heart  of  such  a  solid  stone   as  our  bit  of 
shale.     The  stone  was  once  merely  so  much  sediment 
laid   down  below  water,  and  the  fragmentary  plants 
were  drifted  away  from  the   place   where  they  grew 
until  at  last  they  were  buried  among  that  sediment. 
As  the  mud  hardened  into  shale   the  plant  became 
more  and  more  altered  until  its  substance  passed  into 
coal.     You  will  find  in  a  later  lesson  that  coal  was 
formerly  vegetation  which,  buried  under  great  masses 
of  sediment,  has  been  slowly  changed  into  the  black 
glossy  substance  so  familiar  to  us. 

113.  It  is  not  only  plants,  however,   which   occur 
imbedded  in  sedimentary  rocks.     Here  for  example 
(Fig.  13)  is  a  drawing  of  a  piece  of  shale  in  which 
you  notice  a  number  of  shells  and  other  animal  re- 
mains,   chiefly   trilobites,   that   is,    little  sea-creatures 
belonging  to  the  same  great  tribe  with  our  common 
crab  and  lobster.     You  do  not  need  now  to  be  told 
how  they  came  there.     You  have  learnt  that  anything 
lying  at  the  bottom  of  the  sea  or  of  a  lake  will  be 
buried  in  sediment.     The  remains  of  shells,   corals, 
fishes,  or  any  other  animals  which  live  in  the  water, 


48  SCIENCE  PRIMERS.         [SEDIMENTARY 

must  gather  on  the  bottom  when  these  animals  die, 
and  become  imbedded  in  the  silt  or  other  deposit 
which  is  there  forming.  It  was  clearly  in  this  way 


FIG.  13. — Piece  of  Shale  with  animal  remains. 

that  the  shells  and  corals  in  our  piece  of  shale  were 
preserved. 

114.  Did  you  ever  look  into  the  little  pools  of  sea- 
water  left  upon  a  rocky  beach  when  the  tide  has  gone 
back?     How  full  of  life  they  are  !     Tufts  of  sea-weed 
sprout  up  in  one  place,  groups  of  brightly  tinted  sea- 
anemones  appear  in  another,  periwinkles  and  limpets 
cling  to  the  sides,  and  down  at  the  bottom  you  may 
see  tiny  crabs  cautiously  creeping  out  of  your  sight, 
with  many  other  kinds  of  sea-creatures  moving  to  and 
fro  of  which  you  do  not  know  the  names.    If  you  look 
a  little  more  narrowly  you  can  observe  that  some  of 
the  shells  at  the  bottom  are  empty,  the  animals  which 
once   lived  in   them  having   died,  and   that  broken 
pieces  of  other  dead  creatures  lie  there  also. 

115.  You  are  not  to  suppose  of  course  that  the  whole 


ROCKS.]  GEOLOGY.  49 


of  the  bottom  of  the  sea  is  like  the  bottom  of  one 
of  these  pools  on  the  beach.  The  plants  and  animals 
in  the  pools  are  those  which  live  along  the  shore 
or  shallow  parts  of  the  sea,  while  the  deeper  parts 
have  other  plants  and  animals  peculiar  to  them. 
But  although  these  living  things  differ  greatly  in 
different  portions  of  the  ocean  floor,  and  though 
here  and  there  they  may  be  absent  from  bare  patches 
of  gravel,  stones  or  sand,  the  floor  of  the  great  sea 
resembles  the  floor  of  the  little  pool  on  the  beach  in 
this  respect  that  it  swarms  with  many  kinds  of  living 
creatures,  and  with  the  remains  of  dead  ones.  So  that 
the  deposits  of  sand  and  mud  which  gather  upon 
the  sea-bottom  must  contain  abundant  relics  of  these 
creatures. 

116.  If  then  the  remains  of  plants  and  of  animals 
are  very  generally  buried  in  the  accumulations  of  sedi- 
ment which  now  increase  from  day  to  day  at  the 
bottom  of  lakes  or  of  the  sea,  we  may  be  sure  that 
the  same  must  have  been  the  case  in  past  times,  and 
that  sedimentary  rocks,  which  are  only  so  much  har- 
dened sediment  of  the  bottom  of  old  lakes  or  seas, 
should  also  contain  remains   of  plants  and  animals. 
And  so  they  do  abundantly — you  will  meet  with  sand- 
stones, shales,  and  other  sedimentary  rocks,  as  full  of 
such  remains  as  any  part  of  the  modern  sea-bottorn 
is  now  crowded  with  life. 

117.  Any  relic  of  a  plant  or  animal  imbedded  in 
rock  is  called  a  Fossil.      The   fern  in   Fig.   12  for 
example,  and    the    shells    and   trilobites    in  Fig.    13 
are  fossils.     Some    of    the    questions    which   fossils 
enable  us  to  answer  will  be  pointed  out  in  the  next 
Lesson. 


SCIENCE  PRIMERS.         [SEDIMENTARI 


V.  A  Quarry  and  its  Lessons. 

1 1 8.  ID  the  foregoing  lessons  you  have  learnt  what 
sediment  is,  how  different  kinds  of  sediment,  arranged 
under  water,  have  become  sedimentary  rocks,  and  how 
they  may  contain  the  remains  of  plants  or  animals. 
Let  us  now  try  to  put  some  questions  to  these  rocks, 
and  see  how  they  tell  their  own  story. 

119.  If  you  go  into  the  quarries  which  abound  in 
many  parts  of  this  country  you  may  learn  a  great 
deal  on  this  subject.     Let  us  suppose  ourselves  to  be 
in  such  an  one  as  that  represented  in  Fig.  14. 


FIG.  14.  —  Quarry  in  Sedimentary  R 


120.  In  the  first  place  what  feature  about  the 
quarry  strikes  you  most  forcibly  when  you  enter? 
You  answer  readily,  the  Stratification  of  the 
rocks.  They  are  arranged  in  layers  or  beds,  one 
above  another,  in  that  stratified  arrangement  which 
you  have  found  to  be  so  characteristic  of  rocks  laid 
down  as  sediment  under  water.  (Arts.  90  —  101.) 


ROCKS.]  GEOLOGY.  51 

121.  In  the  second  place,  you  observe  that  they  do 
not  all  consist  of  the  same   materials.     Some  are  of 
fine  conglomerate  (marked  with  little  circles  and  dots 
in  the  drawing),  others  of  various  kinds  of  sandstone 
(marked  with  finer  dots),  and  some  of  different  sorts 
of    shales    or  clays    (marked  with  horizontal  lines). 
These  beds,  or  strata  as  they  are  called,  alternate 
irregularly  with  each  other,  just  as  gravel,  sand,  and 
mud  might  be  found  alternating  in  the  delta  of  a 
river  or  under  the  sea. 

122.  In  the  third  place,  let  me  ask  you  to  point  out 
which  are  the  oldest  of  the  beds.     You  answer  without 
hesitation  that  those  at  the  bottom  of  the  quarry  must 
be  the  oldest  because  they  certainly  were  deposited 
before  those  lying  above  them.     The  lowest  bed  may 
be  of  exactly  the  same  materials  and  thickness  as  one 
or  more  of  the  others,  and  may  so  precisely  resemble 
them  that  you  might  not  be  able  to  see  any  difference 
between  them  if  you  looked  at  them  each  by  itself. 
Yet  their  occurrence  one  above  another  would  prove 
them  not  to  be  the  same  bed,  but  to  have  been  formed 
at  different  times  one  after  the  other.     In  all  such 
cases  the  beds  at  the  bottom  are  the  oldest,  and  those 
at  the  top  the  newest.     This  arrangement  of  one  bed 
or  stratum  above   another   is    called  the  Order  of 
Superposition. 

123.  In  such  a  quarry  as  that  drawn  in  the  woodcut, ' 
this  order  is  no  doubt  very  simple  and  self  evident,  but 
you  will  learn  afterwards  that  it  is  not  usually  so  clear, 
for  in  many  cases  the  rocks  are  concealed  from  you  in 
part  by  soil  or  otherwise,  and  much  care  and  patience 
maybe  needed  before  their  true  order  of  superposition 
is  ascertained.     But  when,  in  spite  of  all  difficulties, 


52  SCIENCE  PRIMERS.         [SEDIMENTARY 

you  succeed  in  showing  which  are  the  bottom  rocks 
and  which  the  uppermost,  you  at  the  same  time  deter- 
mine which  are  oldest  and  which  newest. 

124.  In  the  fourth  place,  let  us  see  if  the  rocks  of 
this  quarry  have  preserved  any  Evidence  as  to  where 
they  were  deposited.  We  split  open  some  of  the  lower 
beds  of  sandstone  and  find  that  their  surfaces  are 
often  covered  with  such  markings  as  are  shown  in  the 
following  drawing  (Fig.  15).  Did  you  ever  see  anything 


FIG.  15.— Ripple-marks  in  Sandstone. 

resembling  these  impressions  elsewhere  ?  If  you  have 
ever  walked  along  a  flat  sandy  beach  you  must  have 
noticed  the  ripple-marks  which  the  shallow  rippling 
water  leaves  on  the  soft  sand.  They  are  precisely  like 
those  on  the  sandstone.  You  may  see  them  too  along 
the  shelving  margin  of  a  lake,  indeed  wherever  water 
has  been  thrown  by  the  wind  into  little  wavelets  over  a 
sandy  bottom.  They  betoken  shallow  water.  Hence 
we  have  learnt  one  important  fact  from  our  quarry,  as 
to  the  origin  of  these  rocks :  viz.  that  they  were  not 
deposited  in  a  deep  sea,  but  in  shallow  water. 


ROCKS.]  GEOLOGY.  53 

125.  We  look  still  further  among  these  strata,  and 
notice  at  last  that  some  of  them  are  curiously  covered 
with  little  round  pits,  about  the  size  of  peas  or  less. 
The  general  appearance  of  these  pitted  surfaces  is 
shown  in  Fig.  16.  How  did  these  markings  come 
there  ?  Like  the  ripple-marks  they  must  of  course  have 
been  impressed  upon  the  sand  when  it  was  soft,  and 
before  it  had  been  hardened  into  sandstone.  Again, 
you  must  seek  for  an  explanation  of  them  by  watching 
what  takes  place  at  the  present  time.  You  know 
that  when  drops  of  rain  fall  upon  a  smooth  surface 


FIG.  16. — Rain-prints  on  Sandstone. 

of  moist  sand,  such  as  that  of  a  beach,  they  each 
make  a  little  dent  on  it.  You  have  learnt  something 
about  these  rain -prints,  and  if  you  compare  the 
present  drawing  with  the  picture  of  the  rain-prints 
in  the  Physical  Geography  Primer,  Fig.  9,  you  will 
see  that  they  are  essentially  the  same,  and  that  they 
have  both  been  made  by  the  fall  of  rain-drops  upon 
soft  moist  sand. 

126.  Here  then  is  another  fact  which  throws  still 
more  light  on  the  history  of  these  rocks.  The  ripple- 
marks  show  that  the  water  must  have  been  shallow  ;  the 


54  SCIENCE  PRIMERS.          [SEDIMENTARY 

rain-prints  prove  that  it  must  have  risen  along  a  beach 
liable,  now  and  then,  to  be  laid  dry  to  the  air  and 
rain.  Now  can  we  tell  whether  the  water  was  salt 
or  fresh?  in  other  words,  was  this  beach  the  shore 
of  a  lake,  or  of  the  sea  ? 

127.  Again  we  turn  to  the  rocks  themselves,  and 
from  some  of  the  layers  of  shale  we  pick  out  a  num- 
ber of  fossils,  which  enable  us  to  answer  the  question. 
If  you  were  to  fish  in  a  lake,  would  you  catch  only  the 
same  fish  which  you  find  in  the  sea?  Certainly  not; 
you  would  soon  learn  that  not  only  the  fishes  but  the 
other  animals  and  the  plants  living  in  fresh  water, 
differ  from  those  living  in  salt  water.  Star-fishes, 
limpets,  oysters,  and  flounders,  for  example,  are  in- 
habitants of  the  sea,  while  your  old  friends  the  perch, 
the  minnow,  and  the  stickleback  are  natives  of  rivers 


FIG.  17.— Fossils,     a,  Coral ;  b,  part  of  Encrinite  ;  c,  Spirifer, 
a  marine  shell. 

and  lakes.  You  can  understand,  therefore,  that  the 
remains  of  animals  and  plants  preserved  in  the  deposits 
of  the  sea-bottom  must  differ  from  those  preserved 
on  the  bottoms  of  lakes. 

128.  Some  of  the  fossils  which  we  have  picked  out 
are  represented  in  the  woodcut  (Fig.  17).     Of  these 


ROCKS.]  GEOLOGY.  55 

a  is  a  coral ;  b  is  part  of  the  jointed  stem  of  the  Encri- 
nite  or  stone-lily — an  animal  related  to  the  common 
star-fish ;  and  c  is  a  shell  belonging  to  a  family  the 
members  of  which  are  all  dwellers  in  the  sea.  Now 
these  are  all  unmistakably  marine  animals,  and  when 
we  find  them  associated  in  this  way  in  a  bed  of  stone, 
we  feel  certain  that  the  materials  of  the  stone  must 
have  been  laid  down  under  the  sea;  they  were  pos- 
sibly cast  ashore  on  the  old  sea  beach,  as  shells  are 
to  this  day. 

129.  Here,  again,  is  a  third  fact  about  the  history 
of  our  rocks.     The  ripple-marks  and  rain-prints  made 
it  certain  that  they  were  formed  in  the  shallow  water 
close   to  shore,  and  along  a  beach ;    and   now  the 
fossils  prove  that  those  waters  were  part  of  the  great 
sea. 

130.  In  this  quarry  then  you  have  found  clear  proofs 
that  land  and  sea  have  here  changed  places.     Though 
the  quarry  may  be  in  the  very  heart  of  the  country, 
far   away  from   the   sea,    yet    you   cannot   be   more 
sure  of  anything  than  that  the  sea  was  once  upon  its 
site.     But  if  you  search  among  other  quarries  you  will 
find  the  same  kinds  of  proofs  of  the  former  presence 
of  the    sea.     In  fact,  were   you   to    start   from   the 
south   of  England,  and  go  north  to  the  far  end  of 
Scotland,  by  much  the   largest  number  of  quarries 
you  would  meet  with  would  be  in  rocks  which  were 
originally  formed  under  the  sea.     In  such  a  journey 
you  would  learn  that  almost  the  whole  of  our  country 
is  made  up  of  such  rocks.     Down  at  the  bottom  of 
deep   mines,   and  away  up  at  the   summits  of  high 
mountains,    you    would    come  upon  them.     Nor   is 
Great  Britain  singular  in  this  respect     Suppose  you 


56  SCIENCE  PRIMERS.  [ORGANIC 

were  to  cross  Europe  and  look  carefully  at  all  the 
rocks  on  your  way,  you  would  still  find  the  sea- 
formed  ones  to  be  the  great  majority.  From  Europe 
into  Asia,  and  from  Asia  through  Africa  on  the  one 
hand,  down  the  whole  length  of  America  on  the 
other,  you  would  encounter  far  more  rocks  which 
had  been  formed  under  the  sea,  than  of  any  other 
kind.  The  very  highest  mountains  in  the  world  con- 
sist of  sea-made  rocks. 

131.  Now  is  this  not  a  very  singular  fact?    How  is 
it  that  the  solid  land  has  been  chiefly  made  under  the 
sea?     The  rocks  must  have  been  raised  up  out  of 
the  sea  by  some  means,  and   since  the  land  is  so 
uneven  they  would  seem  to  have 'been  raised  much 
more    in    some   places    than   in    others.       How  this 
raising  of  the  sea-bed  has  taken  place,  will  be  spoken 
of  in  a  later  lesson.      But  first  we  must  trace  the 
history  of  certain  other  rocks,   many  of  which  have 
also  been  formed  under  the  sea. 

ORGANIC  ROCKS,  OR  ROCKS  FORMED  OF  THE 
REMAINS  OF  PLANTS  AND  ANIMALS. 

I.  Rocks  formed  of  the  Remains  of  Plants. 

132.  Since  the  leaves,  branches,  and  stems  of  plants, 
and  the  shells  or  other  remains  of  animals,  are  some- 
times scattered  so  abundantly  through  ordinary  sedi- 
mentary rocks,  it  is  easy  to  see  that  sometimes  they 
may  occur  in  such  quantity  as  to  form  great  deposits 
of  themselves.     You  could  hardly  call  such  deposits 
sedimentary,  in  the  same  sense    in  which    common 
shale  and  sandstone  are  so  named.     We  may  term 
them  Organic   Rocks,  or,  Organically  derived 


ROCKS.]  GEOLOGY.  57 

Rocks,  because  they  owe  their  origin  to  the  accumu- 
lation of  what  are  called  organic  remains,  or  fossils, 
that  is,  the  remains  of  plants  or  animals.  A  plant  or 
animal  lives,  moves,  and  grows  by  means  of  what  are 
called  organs.  For  instance,  we  walk  by  using  our 
legs,  which  are  OUT  organs  of  locomotion  ;  we  speak  with 
our  mouth,  which  contains  our  organs  of  speech ;  we  see 
by  means  of  eyes,  which  are  our  organs  of  sight ;  and  so 
on.  Every  object,  therefore,  which  possesses  organs  is 
said  to  be  organized  or  to  be  an  organism.  So  that 
when  you  see  this  word  organism  you  will  remember 
that  it  means  either  a  plant  or  an  animal,  for  it  is  only 
plants  and  animals  which  are  really  organized. 

133.  We  begin  with  those  rocks  which  have  been 
formed  out  of  the  remains  of  plants.     As  an  illustra- 
tion let  me  ask  you  to  examine  carefully  a  piece  of 
coal.     If  you  master  all  that  it  has  to  tell  you,  you 
will  not  have  much  difficulty  in  tracing  out  the  history 
of  other  rocks  belonging  to  this  series. 

134.  You  know  well  the  general  appearance  of  coal. 
Did  you  ever  notice  that  though  brought  to  the  fire- 
place in  rough,  irregular  lumps,  it  has  nevertheless 
an  arrangement  in  layers  like  the  sedimentary  rocks  ? 
Try  to  break  a  big  solid  piece  of  coal,  and  you  find 
that  it  usually  splits  more  easily  in  one  direction  than 
in  any  other.     This  direction  is  that  of  the  thin  layers 
of  which  the  coal  consists.    If  you  want  large  pieces 
of  coal  to  burn  up  quickly  and  make  a  good  fire,  you 
will  take  care  so  to  put  them  in  the  grate  that  those 
layers  shall  be  more  or  less  upright.     In  that  position 
the  heat  splits  them  up. 

135.  Now  look  at   one  end  of  a  lump  of  coal, 
where   the  edges  of  the   layers  are   exposed.     You 


58  SCIENCE  PRIMERS.  [ORGANIC 

cannot  follow  them  with  the  same  ease  as/  in  the 
case  of  a  piece  of  shale,  for  they  seem  to  blend  into 
one  another.  But  you  may  notice  that  among  the 
layers  of  hard,  bright,  glossy  substance,  there  occur 
others  of  a  soft  material  like  charcoal.  A  mere  general 
look  at  such  a  piece  of  coal  would  show  you  that  it 
is  stratified. 

136.  You  know  that  coal  can  be  burnt  away  so  as 
to  leave  only  ashes  behind,  and  that  in  this  respect  it 
resembles  wood  and  peat  (see  Art.  145).     Chemists 
have  analysed  coal  and  found  that  it  consists  of  the 
same  materials  as  wood  or  peat,  and  that  in  reality 
it  is  only  so  much  vegetation  which  has  been  pressed 
together,  and  gradually  changed  into  the  black  sub- 
stance now  used  as  fuel. 

137.  Let  us  suppose  ourselves  at  a  coal-mine,  with 
the  object  of  seeing  exactly  how  the  coal  lies  before  it 
is  dug  out  of  the  earth  and  broken  up  into  the  small 
pieces  which  we  burn  in  our  grates  (see  Fig.  37).     We 
descend  in  one  of  the  cages  by  which  the  miners  are 
let  down  into  the  pit.    After  our  eyes  have  got  a  little 
used  to  the  darkness  at  the  bottom,  we  set  out,  lamp 
in  hand,  along  one  of  the  roadways,  and  reach  at  last 
a  part  of  the  pit  where  the  miners  are  at  work  re- 
moving the  coal.      Now,  first  of  all,  you   see   that 
the  coal  occurs  as  a  bed,  having  a  thickness  of  a 
few  feet     This  bedded  character  agrees   with   what 
you  have  already  noticed  as  to  the  internal  layers  in 
the  stone,  and  confirms  you  in  believing  that  coal  is 
a  stratified  rock.     Next  observe  that  the  pavement  on 
which  the  coal  rests,  and  the  roof  which  covers  it, 
are  both  made  of  very  different  materials  from  the 
coal  itself.     Were  you  to  cut  a  trench  or  section  (Art 


ROCKS.]  GEOLOGY.  59 

90)  through  pavement,  coal,  and  roof,  you  would 
find  some  such  arrangement  as  in  Fig.  18.  You  would 
prove  beyond  any  doubt  that  the  bed  of  coal  lies 
among  beds  of  common  sedimentary  rock. 

138.  But  what  is  this  layer  marked  <£,  forming  the 
floor  or  pavement  on  which  the  coal  lies  ?  Examine  it 
with  attention  and  you  recognize  it  to  be  a  bed  of 
dark  clay,  with  abundance  of  black  streaks  and  branch- 
ing strings,  like  roots,  spreading  through  it.  You  may 
trace  these  root-like  strings  into  the  bottom  of  the  very 


F:G-18,-— Section  of  a  Coal-seam  w"h  its  roof  and  pavement,  a.  Sandstones, 
bhales  &c.  b.  Under-day  forming  pavement  of  Coal  (c).  d.  Sandstones 
and  Shales,  forming  roof  of  Coal 

coal  itself.  If  you  visited  other  pits  you  would  find  each 
coal-seam  to  lie  usually  on  such  a  bed  as  this.  Now 
why  should  the  coal  rest  rather  on  a  bed  of  clay 
or  shale  than  on  one  of  sandstone  or  any  other  sort 
of  rock?  If  you  noticed  that  this  peculiar  pavement 
met  you  in  every  pit  you  visited,  would  you  not 
begin  to  feel  quite  sure  that  the  constant  association 
of  the  coal  and  its  under-clay  could  not  be  a  mere 
accident  but  must  have  a  meaning? 

139.  Now  look  at  the  under-clay  again.      Does  it 
not  remind  you  of  a  bed  of  soil  with  roots  branching 


60  SCIENCE  PRIMERS.  [ORGANIC 

through  it  ?  With  this  idea  suggested  to  your  mind, 
the  more  you  examine  the  rock  the  clearer  will  this 
resemblance  appear,  until  you  are  driven  to  conclude 
that  in  truth  the  under-day  is  an  old  soil,  and 
the  bed  of  coal  represents  the  vegetation 
which  grew  upon  it.  (See  Fig.  38.) 

140.  Each  coal-seam  has  been  in  truth  at  one  time 
a  dense  mass  of  vegetation  growing  on  a  wide  marshy 
flat,  somewhat  like   the  swampy  jungles  of  tropical 
countries  at  the  present  day.     These  great  marshy 
plains  had  a  bottom  of  muddy  soil  on   which  the 
rank  vegetation  grew,  and  it  is   this  very  soil  which 
you  still  see  in  the  under-clay. 

141.  Can  we  tell  anything  about  the  kind  of  plants 
which  flourished  over  these  plains,  and  accumulated 
into  the  thick  mass  which  formed  the  coal  ?    Not  much 
can  usually  be  made  out  from  the  coal  itself,  for  the 
vegetation   has  been  so  squeezed  and  altered  as  to 
destroy  the  leaves  and  branches  of  the  plants;   yet 
in  many  kinds  of  coal  parts  of  the  old  plants  have 
been  changed  into  a  sort  of  charcoal,  which  soils  the 
finger,  and  shows  traces  of  the  vegetable  fibre  like 
any  ordinary  charcoal.     If  you  cut  slices  from  coal, 
fix   these  on   glass,  rub    them    down  until  they  are 
so  thin  as  to  be  transparent,  and  place  them  under  a 
microscope,  you  may  often  find  that  the  coal  contains 
millions  of  little  seed-vessels,  or,  as  they  are  called, 
sporangia.     These  were  shed  by  plants  somewhat 
like  the  club-mosses  of  our  own  moors  and  hills,  but 
much  larger  in  size,  and  must  have  fallen  so  thickly 
over  the  flat  grounds  as  to  form  a  kind  of  mould  or 
soil  upon  them. 

142.  But  though  the  larger  plants  have  not  usually 


ROCKS.  ] 


GEOLOGY. 


61 


been  preserved  well  in  the  coal  itself,  you  may  some- 
times find  them  in  great  perfection  and  beauty  in  the 
beds  of  rock  above  or  below  the  coal.  Some  of  the 
common  varieties  are  shown  in  Fig.  19.  Now  and 
then  you  may  see  these  plants  lying  across  each  other 


FIG.  19.— Plants  out  of  which  Coal  has  been  formed. 

and  all  squeezed  flat,  but  still  retaining  much  of  their 
original  gracefulness,  upon  the  bottom  of  the  bed 
of  rock  which  forms  the  roof  of  the  galleries  as 
you  go  through  the  coal-mine. 

143.  Each  coal-seam,  once  a  luxuriant  surface  of 
vegetation,  open  to  the  sunlight,  and  stretching  over 
many  square  miles,  now  lies  buried  deep  within  the 
earth,  under  huge  masses  of  rock,  which  must  be 
bored  through  before  the  coal  can  be  reached.  How 


62  SCIENCE  PRIMERS.  [ORGANIC 

this  burying  has  taken  place  we  shall  find  out  in  a 
later  lesson  (Arts.  213 — 216).  In  the  meantime  you 
should  learn  a  little  about  another  kind  of  formation, 
where  vegetation  comes  into  play,  and  which  you  may 
examine  not  in  a  deep  mine  but  in  the  open  day. 

144.  You  have  no  doubt  read  about,  you  may  even 
have  seen,  the  bogs  and  peat-mosses  so  abundant  in 
Ireland,  Scotland,  and  some  parts  of   England.      If 
you  have  not,  you  must  imagine  a  wide,  flat  space  of 
brown  moor  and  green  marsh,  in  many  parts  so  soft 
and  wet  that  you  would  sink  deep  into  the  black 
mire  if  you  tried  to  walk  on  its  treacherous  surface ; 
in   other   parts  having  a  firmer  crust,  which   shakes 
under  your  feet  as  you  jump  from  one  dry  standing- 
place  to  another.     Such  a  flat  space  is  called  a  bog 
in    Ireland,  whilst   in   Scotland    and    England   it   is 
known   as   a  moss,   or  peat-moss.       Of   the   whole 
surface  of  Ireland  nearly  a  seventh  part  is  believed 
to  be  occupied  with  bogs,  and  in  many  parts  of  Scot- 
land too  they  occur  in  great  numbers. 

145.  Visiting  one  of  these  places  you  notice  that 
round  its  edges  it  is  usually  quite  firm.      It  may  even 
have  become  so  dry  over  the  very  centre  as  to  be 
ploughed  up  and  to  furnish  crops  of  turnips  and  pota- 
toes.   Wherever  you  can  catch  a  sight  of  the  substance 
of  which  the  moss  consists,  you  find  it  to  be  a  black 
or  dark  brown  sort    of   mould    called  Peat,   formed 
of  the  remains    of    plants    firmly   matted   together. 
Over  the  whole  of  the  moss  this  peat  extends  as  a 
bed,    sometimes   thirty   or   forty   feet    thick.      It    is 
simply  a   vegetable  deposit,  and    in    this  and   other 
respects  resembles  coal. 

146.  Such  being  its  composition  it  may  of  course 


ROCKS.] 


GEOLOGY. 


be  readily  burnt,  so  that  at  the  mosses  it  is  dug  out  in 
pieces,  which  are  dried  and  used  for  fuel.  Over  great 
parts  of  Ireland  and  wide  regions  of  Scotland  the 
peasantry  have  no  other  fuel  than  this  peat,  which 
they  cut  every  summer  from  the  mosses. 

147.  In  Fig.  20  a  representation  is  given  of  one  of 
these  cuttings  for  peat.  It  is  in  such  places  that  the 
mode  of  origin  of  the  deposit  can  best  be  studied, 
and  as  the  tracing  out  of  the  formation  of  a  peat-moss 


FIG.  20.— Section  of  a  Peat  moss,  where  the  peat  is  cut  and  piled  into 
small  stacks  to  dry  for  fuel. 

furnishes  a  good  example  of  the  way  in  which 
geologists  try  to  find  out  the  past  history  of  the 
earth,  let  me  ask  you  to  suppose  yourselves  looking 
into  the  opening  which  has  been  made  in  the  peat- 
moss drawn  in  Fig.  20. 


64  SCIENCE  PRIMERS.  [ORGANIC 

148.  Below  the  surface  of  coarse  grass  and  heather 
lies  the  peat,  a  brown  fibrous  mass  in  the  upper  parts, 
but   getting  more   and   more   compact   towards  the 
bottom,  till  it  passes  perhaps  into  a  dark  compact 
substance  in  which  no   trace  of   any  fibres  may  be 
discernible.      Down  below  the  bottom  of   the  peat 
there  sometimes  lies  a  layer  of  fine  clay,  containing 
the  remains  of  shells  which  are  only  found  living  in 
fresh   water.       Now   and   then,   too,   a    rude    canoe, 
hollowed  out  of  the  trunk  of  an  oak  tree,  is  dug  up 
from  the  bottom  of  a  peat-moss — a  relic  of  some  of 
our  uncivilized  ancestors. 

149.  Here,  then,  is  a  little  bit  of  geological  history. 
Now  put  these  separate  facts  together  and  make  out 
the  story  of  the  peat- moss. 

150.  Beginning  at  the  bottom,  the  oldest  formation 
you  meet  with  is  the  layer  of  clay  just  referred  to.   You 
have  already  learnt  that  such  a  layer  must  have  been 
laid  down  under  water.     If  it  should  happen  to  be 
thick  it  will  suggest  to  you  that  probably  this  water 
was  not  a  mere  shallow  pool  or  brook,  but  had  some 
depth  and  extent.    But  the  shells  indicate  further  that 
the  water  must  have  been  that  of  a  lake,  for  they  are 
such  shells  as  you  might  find  still  living  in  the  lakes 
of   the  neighbourhood.      The  first  point  you  settle, 
therefore,  is  that  before  a  peat-moss   existed  here,  a 
lake  occupied  its  site.     You  may  even  yet  trace  what 
the  boundaries  of  this  lake  were,  for  the  slopes  which 
rise  all  round  the  flat  peat-moss  must  in  the  same 
way  have  surrounded  the  old  sheet  of  water,  whereon 
our  rude  forefathers  floated  the  canoes  which  are  now 
and  then  dug  up  from  the  bottom  of  the  mosses. 

151.  Above  the  layer  of  clay  which  marks  the  former 


ROCKS,  j 


GEOLOGY. 


lake-bottom,  comes  the  bed  of  peat,  made  up  wholly 
of  vegetable  materials.  Evidently  it  has  taken  the 
place  of  the  water.  The  plant-remains  have  filled 
the  shallow  lake  up,  and  converted  it  into  a  peat- 
moss. In  many  places  you  may  see  this  process 
actually  going  on  still.  In  such  a  peat-moss,  for  ex- 
ample, as  that  shown  in  Fig.  21,  it  is  evident  that  the 
little  patch  of  water  in  the  centre  is  only  a  remnant 
of  the  lake,  which  once  covered  the  whole  hollow. 


FIG   21.— Ground-plan  or  map  of  a  Peat-moss  filling  up  a  former  lake,  and 
w.th  a  portion  of  the  lake  still  unfilled  up. 

At  the  edge  of  that  remaining  pool  you  find  that  the 
marshy  vegetation  out  of  which  the  peat  has  been 
formed  is  growing  into  the  water  on  all  sides.  Put  a 
pole  down  to  the  bottom  and  you  will  stir  up  the  fine 
black  or  brown  peat,  formed  out  of  decayed  roots 
and  fibres.  Here  there  is  still  some  water  between 
the  dead  peaty  matter  at  the  bottom  and  the  grow- 
ing plants  which  form  a  sort  of  crust  over  the  top. 
But  in  the  end  the  plants  will  fill  up  the  whole  of 


66  SCIENCE  PRIMERS.  [ORGANIC 

this  intermediate  space,  and  then  even  the  centre 
will  be  converted  into  a  solid  bed  of  peat,  as  all  the 
outer  parts  of  the  moss  have  already  been. 

152.  Peat-mosses    have    been  formed    in   marshy 
grounds  or  shallow  lakes  by  the  growth  and  decay 
of  plants,   and  the    accumulation    of  their  remains 
on  the  place  where  they  lived  and  died.    Like  coal- 
seams  they  show  how   in  certain  circumstances  the 
growth  and  decay  of  plants  may  give  rise  to  thick 
and  wide-spread  deposits. 

II.  Rocks   formed  out  of  the  Remains  of 
Animals. 

153.  At  first  when  you  think  of  it,  there  seems  not 
much  chance  of  animal  remains  accumulating  to  such 
a  depth  as  to  form  any  well-marked  deposit.    Though 
the  air  may  be  filled  with  insects,   though  birds  in 
abundance  may  be   seen  and  heard  as  the  summer 
slips  away,  though  in  our  meadows  and  woodlands 
rabbits,  hares,  moles,  and  many  other  creatures  live 
in  great  numbers,  yet  you  nowhere  see  their  remains 
forming  a  deposit  on   the   surface.     Nay,   you  com- 
paratively seldom   see  a  dead  animal  at  all.     They 
creep   into   holes   and  die    there,   and    their   bodies 
gradually    crumble    away    and    disappear.      But    if 
you  look  at  the  right  places  you  will  discover  that 
the-   remains    of  animals   as  well  as  of  plants,  and 
indeed  much  more  than  plants,  form  great  accumu- 
lations. 

154.  In   the  bed  of  clay  under  a   peat-moss,  as 
described  in  Art.  148,  the  shells  which  are  sometimes 


ROCKS.]  GEOLOGY.  67 

to  be  seen  mouldering  away  belong  to  certain  kinds 
which  live  in  lakes.  In  some  parts  of  the  country  the 
bottoms  of  the  lakes  are  covered  with  similar  shells, 
so  much  so  that  if  you  were  to  take  a  boat  and  begin 
to  dredge  up  some  of  the  soft  mud  from  the  bottom 
of  one  of  these  sheets  of4  water,  you  would  find  it  to 
consist  of  a  kind  of  white  chalky  substance,  or  marl 
as  it  is  called,  made  up  of  shells  in  all  stages  of  decay. 
The  animals  which  live  in  these  shells  so  abound  in 
the  water  that  as  they  die  their  shells  form  a  layer 
over  the  floor  of  the  lake.  Now  and  then  such  a  lake 
has  been  either  gradually  filled  up  by  being  choked  with 
vegetation  and  silt  (Art.  151),  or  has  been  drained 
artificially  so  as  to  be  converted  into  dry  land. 
Digging  down  on  the  site  of  that  vanished  lake 
you  would  come  to  the  fresh-water  marl,  forming  a 
bed  or  layer  several  feet,  or  even  yards,  in  thick- 
ness. Perhaps  you  would  meet  with  the  skeleton 
of  some  deer,  or  wild  ox,  or  other  animal,  which 
had  somehow  been  drowned  in  the  old  lake  ;  or 
you  might  disinter  the  canoe  or  stone-hammer  or 
other  relic  of  the  early  human  races,  which  peopled 
the  country  before  so  many  of  its  lakes  and  forests 
had  disappeared.  In  some  districts  where  limestone 
is  scarce,  the  marl  of  the  old  lakes  has  been  dug  up 
in  large  quantities  as  a  manure  for  the  land.  Hence 
you  learn  that  even  the  frail  shells  which  are  to  be 
seen  on  the  stones  and  reeds  along  the  margin  of  a 
lake  may  afford  an  illustration  of  how  rocks  are  formed 
out  of  the  remains  of  animals. 

155.  It  is  on  the  floor  of  the  great  sea,  however, 
that  the  most  wonderful  examples  occur  of  the  way  in 
which  rocks  are  gradually  built  up  from  the  remains 
7 


68  SCIENCE  PRIMERS.  [ORGANIC 

of  animals  to  a  depth  of  many  hundreds  or  thousands 
of  feet,  and  over  distances  of  many  hundreds  of 
miles.  Something  has  already  been  said  on  this 
subject  in  the  Physical  Geography  Primer,  Arts.  236 
and  247  ;  where  the  use  of  the  dredge  for  the  ex- 
ploration of  the  bottom  of  the  ocean  was  referred 
to,  and  allusion  was  made  to  the  fine  mud,  formed 
of  minute  organic  remains,  and  found  over  most  of 
the  bed  of  the  Atlantic  Ocean.  Let  us  now  consider 
this  mud  a  little  further. 

156.  To  the  west  of  Britain  the  Atlantic  soon  and 
suddenly  deepens.  Its  floor  then  stretches  away  to 
Newfoundland  as  a  vast  plain,  the  lowest  part  of 
which  is  about  14,000  feet  below  the  waves.  It  was 
over  this  wide  sub-marine  plain  that  the  Telegraph- 
cables  had  to  be  laid,  and  hence  numerous  soundings 
were  made  all  the  way  across  from  Ireland  to  the 
American  coast  (Physical  Geography  Primer,  Art. 
234).  While  in  the  shallower  parts  of  the  sea  the 


FIG.  22.— Some  of  the  Ooze  from  the  Atlantic  bed,  magnified  25  times. 


bottom  was  found  to  be  covered  with  sand,  gravel, 
or  mud,  from  the  deeper  parts  there  came  up  with 
the  sounding-lead  a  peculiar  grey  sticky  substance 
known  as  ooze,  which  must  stretch  over  that  wide 


ROCKS.  J  GEOLOGY.  69 

deep-sea  basin  for  many  thousands  of  square  miles. 
This  ooze  when  dried  looks  like  a  dirty  kind  of  chalk. 
You  may  purchase  a  minute  quantity  of  it  prepared 
on  a  glass  slide  for  the  microscope.  Looking  at  such 
a  slide  with  only  your  naked  eyes,  you  might  suppose 
that  the  little  specks  you  see  are  merely  so  many  grains 
of  dust  upon  the  glass.  But  place  them  under  a  strong 
magnifying  glass  or  microscope,  and  you  discover  that 
they  consist  of  minute  shells  called  Foraminifera,  some 
of  them  quite  entire,  others  broken,  and  all  most  deli- 
cately sculptured  and  punctured  (Fig.  22).  As  you  look 
at  these  graceful  forms,  reflect  that  they  are  crowded 
together,  millions  upon  millions,  over  the  floor  of  the 
Atlantic,  that  as  they  die  their  shells  gather  there 
into  a  wide-spread  deposit,  and  that  as  fresh  gene- 
rations spring  up  one  after  another  this  deposit  is 
continually  getting  deeper.  After  the  lapse  of  cen- 
turies, if  the  deposit  were  to  remain  undisturbed, 
and  if  we  could  set  a  watch  to  measure  its  growth, 
we  should  find  it  to  have  risen  upward  and  to  have 
enclosed  the  remains  of  any  star-fishes  or  other  sea- 
creatures  which  chanced  to  die  and  leave  their  re- 
mains upon  the  bottom.  Hundreds  of  feet  of  such 
slow-formed  deposit  have  no  doubt  already  been 
laid  down  over  the  bottom  of  the  ocean  between 
Ireland  and  Newfoundland.  Here  then*  is  a  second 
and  notable  example  of  how  a  deep  and  far-spread 
mass  of  rock  may  be  formed  out  of  the  remains  of 
animals. 

157.  Now  return  once  more  to  our  piece  of  chalk 
(Art.  28)  and  compare  it  with  the  ooze  of  the  Atlantic. 
At  the  first  glance  in  many  a  piece  of  chalk  you 
can  see  shells,  corals,  sea-urchins,  and  other  marine 


70  SCIENCE  PRIMERS.  [ORGANIC 

remains,  either  entire  or  in  fragments  (Fig.  23).  These 
are  enough  to  convince  you  that  chalk  must  have  been 
formed  under  the  sea.  But  a  little  further  examination 
will  show  that  the  chalk  not  merely  contains  animal 
remains,  but  is  altogether  made  up  of  them.  If  you 
were  fortunate  in  the  piece  of  chalk,  which  you  treated 
as  recommended  in  a  former  Lesson  (Art.  28),you  found 
numerous  little  cases  or  shells  (Fig.  3),  quite  like  those 
in  the  Atlantic  ooze  (Fig.  22),  along  with  fragments 
of  larger  broken  shells  and  other  remains.  The  whole 


FIG.  23.— A  piece  of  Chalk  with  shell  in  it 

of  the  chalk  is  evidently  made  of  animal  remains,  some 
quite  perfect,  others  so  broken  and  crumbled  that  you 
cannot  be  sure  to  what  kind  of  sea-creatures  they 
belonged.  You  must  not  be  disappointed  if  for  a 
time  none  of  the  chalk  which  you  brush  off  shows 
you  any  distinct  organism  (Art.  132),  but  only  shape- 
less white  grains.  All  these  grains  are  only  the 
mouldered  fragments  of  organisms,  and  you  must 
search  among  them  until  you  find  some  still  perfectly 
preserved  and  entire  specimens.  When  successful  you 
will  meet  with  some  such  assemblage  of  minute  organic 


ROCKS.]  GEOLOGY. 


remains  as  shown  in  Fig.  3,  which  represents  some  of 
the  chalk  of  Gravesend. 

158.  But  chalk  is  only  one  of  many  rocks  composed 
altogether  of  the  remains  of  animals.  Most  of  the 
limestones  have  been  formed  out  of  these  materials. 
Here,  for  instance,  is  a  piece  of  limestone  (Fig.  24) 
which  has  been  lying  exposed  to  the  air  for  many 


FIG.  24  — Fiece  of  Limestone,  showing  how  the  stone  is  made  up  of  animal 


years,  and  you  see  how  its  surface  is  crowded  with 
bits  of  "  stone-lilies,"  corals,  shells,  and  other  re- 
mains. The  sight  of  such  a  piece  of  stone  as  this 
at  once  sets  you  thinking  about  some  old  sea-floor. 
You  can  picture  to  yourselves  how  all  these  delicately 
sculptured  little  fragments  once  formed  parts  of  living 
creatures,  which  moved  or  grew  beneath  the  clear 
waters  of  the  sea.  The  bit  of  limestone  becomes  to 


72  SCIENCE  PRIMERS.  [ORGANIC 

you  a  kind  of  model  of  what  a  sea-floor  must  be,  and 
reminds  you  of  what  you  may  even  have  seen  with 
your  own  eyes  at  the  bottoms  of  some  of  the  rocky 
pools  upon  the  beach.  (Art.  114.) 

159.  If  a  little  fragment  of  limestone  might  suggest 
these  thoughts  to  you,  what  would  you  think  if  you 
were  taken  to  places  where  all  the  hills  are  made  up 
of  such  limestone — vast  piles   of  rock  two  or  three 
thousand  feet  thick,  and  stretching  over  the  land  for 
hundreds  of  square  miles  ?  And  yet  you  may  meet  with 
such  wonderful  masses  of  limestone  crowded  with  the 
remains  of  old  sea-creatures,  in  almost  every  country  of 
the  world.     In  Britain,  for  example,  the  hills  and  dales 
of  a  great  part  of  Derbyshire  and  Yorkshire  are  built 
up  of  limestone.     Looking  up  one  of  these  wonderful 
valleys  you  see  the  beds  of  limestone  winding  along 
either  side  and  rising  in  broad  terraces,   one  above 
the  other  as  far  as  the  eye  can  reach.     In  walking 
along  the  surface  of  one  of  these  high  hill-terraces, 
you  are  really  walking  on  the  bottom  of  an  old  sea, 
and  if  you  stop  anywhere  to  look  at  the  rock  under 
your  feet,  you  will  see  that  it  is  only  a  mass  of  the 
crowded  remains  of  the  little  animals  which  peopled 
the  waters  of  that  sea.     Somehow   the  sea-bed  has 
become  dry  land,  and  the  thick  animal  deposits  of  its 
bottom  have  hardened  into  limestone,  out  of  which 
high  hills  and  wide  valleys  have  been  formed. 

1 60.  Still  thicker  masses  of  similar  limestone  occur 
in  Ireland.     Some  of  the  giant  mountain  chains  of  the 
world  consist  in  great  measure  of  limestone.     Among 
the  lofty  crests  of  the  Alps,  for  example,  and  in  the 
chain  of  the  Himalaya,  limestone,  made   up  of  the 
remains  of  marine  animals,  is  found  to  constitute  great 


ROCKS.]  GEOLOGY.  73 

ranges  of  the  high  ground  on  which  the  eternal  snows 
rest  and  from  which  the  glaciers  descend  into  the 
valleys. 

161.  Summary.    Before  advancing   further  you 
may  now  look  back  upon  what  you  have  learnt,  and  see 
exactly  the  point  to  which  you  have  come.     If  I  were 
to  ask  you  to  make  a  short  abstract  of  the  foregoing 
lessons  you  would  probably  jot  down  such  a  summary 
as  the  following  : — 

(i.)  The  surface  of  the  land  is  worn  away  by  rain 
and  by  streams,  and  a  vast  deal  of  mud,  sand,  and 
gravel  is  consequently  formed. 

(2.)  This  material  worn  from  the  land  accumu- 
lates at  the  mouths  of  rivers,  in  lakes,  and  over  the 
floor  of  the  sea,  so  as  to  form  great  deposits,  which 
will  in  the  end  harden  into  Sedimentary  Rocks. 

(3.)  Leaves,  twigs,  trunks,  and  other  parts  of  plants, 
together  with  the  remains  of  animals,  become  im- 
bedded and  preserved  as  Fossils  in  these  sedimentary 
accumulations. 

(4.)  Plants  and  animals  of  themselves  sometimes 
form  thick  and  extensive  deposits  upon  the  surface 
of  the  earth. 

(5.)  The  rocks  of  which  the  dry  land  is  made  have 
been  formed,  for  the  most  part,  under  the  sea. 

(6.)  Old  land-surfaces  which,  like  the  coal-seams, 
once  spread  out  into  luxuriant  forests,  are  now  buried 
deep  beneath  the  present  surface  under  masses  of 
solid  rock. 

162.  You  have  advanced  step  by  step  to  these  con- 
clusions, and  are  quite  sure  of  them,  for  you  have 
tested  everything  on  the  way.     Again  and  again  you 
have  met  face  to  face  with  proofs  that  in  some  way 


74  SCIENCE  PRIMERS.  [IGNEOUS 

or  other  land  and  sea  have  often  changed  places. 
You  have  found  old  sea-bottoms  even  up  among  the 
crests  of  the  mountains.  You  have  found  old  forests 
buried  in  the  form  of  coal-seams  deep  in  the  bowels  of 
the  earth.  How  can  these  wonderful  changes  have 
taken  place  ?  To  be  able  to  answer  this  question  you 
must  find  out  something  about  the  history  of  the  third 
of  the  three  great  groups  into  which  we  divided  the 
stones  of  the  earth — the  Igneous  Rocks. 

IGNEOUS    ROCKS. 
I.  What  Igneous  Rocks  are. 

163.  Turning  back  to  one  of  the  early  Lessons  of 
this  book  (Art.  44),  you  find  that  we  divided  stones 
into  three  great  classes,  of  which  the  third  was  named 
the  Igneous  class.     This  word  igneous,  means  literally 
fiery.     It  does  not  very  accurately  describe  the  rocks 
to  which  it  is  applied,   but  it  has  long  been  in   use 
to  include  all  rocks  which  have  been  actually  melted 
within  the  earth,  or  which  have  been  thrown  out  at  the 
surface  by  the  action  of  volcanos.    So  that  the  Igneous 
Rocks  owe  their  origin  to  some  of  the  effects  of  the 
internal  heat  of  the    earth,    about   which   you   have 
already  learnt  something  (Physical  Geography  Primer, 
Arts.  252 — 265),  and  must  now  learn  more. 

164.  The  first  thing  to  occur  to  you  when  you  begin 
to  search  for  examples  of  igneous  rocks,  will  probably 
be  the  fact  that  they  are  by  no  means  so  abundant 
as  the  other  two  great  classes  of  rocks.     Take  Great 
Britain    as    an    illustration.     If    you    traversed    the 
country  from  end  to  end,  you  would  meet  everywhere 
with  rocks  belonging   to   the    Sedimentary,    and   the 


ROCKS.]  GEOLOGY.  75 

Organic  series.  But  you  would  travel  over  con- 
siderable spaces  without  meeting  with  any  of  the 
Igneous  class.  The  whole  of  that  part  of  England, 
for  example,  which  lies  to  the  south-east  of  a  line 
drawn  from  Lyme  Regis,  by  Leicester,  to  Flamborough 
Head,  contains  not  a  single  mass  of  igneous  rock. 
And  yet  were  you  to  cross  over  into  North  Wales,  or 
Cumberland,  or  the  midland  valley  of  Scotland,  you 
would  find  rocks  of  that  kind  in  abundance,  protruding 
through  the  surface,  and  forming  many  of  the  highest 
and  most  picturesque  hills  and  crags  in  these  parts  of 
the  island.  So  that  though  igneous  rocks  are  not 
universally  diffused,  they  occur  abundantly  enough 
in  many  places.  Even  in  so  small  a  space  as  Britain 
they  are  plentiful ;  they  are  likewise  to  be  met  with  in 
most  parts  of  the  world.  They  have  a  very  curious  and 
important  history,  and  hence  it  is  desirable  that  you 
should  know  what  they  really  are,  and  how  you 
could  recognize  them. 

165.  In  the  account  given  of  volcanos  in  the  Physical 
Geography  Primer  (Art.   258)   you  will  find  that  the 
solid  materials  cast  up  by  volcanos  were  stated  to  be 
of  two  kinds — ist,  streams  of  molten  rock  called  lava 
poured    down    the    sides    of    a    volcanic    mountain 
during  an  eruption ;    and  2nd,  immense  quantities  of 
dust,  sand,  and  stones,  cast  up  into  the  air  from 
the  mouth  of  the  volcano,  and  falling  down  upon  the 
mountain,  sometimes  even  all  over  the  surrounding 
country  for  a  distance  of  many  miles. 

1 66.  Here  then  are  two  very  dissimilar  kinds  of  rock- 
material  discharged   from   the  interior  of  the  globe. 
The  lava  cools  and  hardens  into  a  solid  rock.     The 
loose  ashes  and  stones,  likewise,  are  in  time  pressed 


76  SCIEXCE  PRIMERS.  [IGNEOUS 


and  hardened  into  more  or  less  firm  beds  of  stone.  So 
that  two  totally  distinct  kinds  of  rock  are  laid  down 
upon  the  surface  of  the  earth  by  the  volcano.  In  the 
case  of  the  lava,  the  rock,  if  you  look  at  it  with  a 
magnifying  glass,  is  seen  to  be  made  up  of  distinct 
crystals  all  matted  together.  The  beds  of  ashes,  on 
the  other  hand,  no  matter  how  compact  they  may 
have  become,  are  found  to  be  made  up  of  irregu- 
lar fragments  of  various  kinds  of  stone,  and  of  all 
sizes,  from  the  finest  dust  up  to  big  blocks.  By 
attending  to  this  very  simple  and  intelligible  differ- 
ence you  could  arrange  igneous  rocks  into  two  great 


FIG.  25.—  Piece  of  Lava,  showing  the  crystals  and  the  steam-holes. 


groups — ist,  the  Crystalline,  that  is,  those  which 
are  made  up  of  crystals,  and  which  have  once  been 
in  a  melted  state ;  and  2nd,  the  Fragmental,  that 
is,  those  which  consist  of  the  loose  materials  thrown 
out  during  volcanic  explosions. 

167.  i.  Crystalline  Igneous  Rocks.  The 
piece  of  Granite  which  we  have  examined  (Art.  26) 
is  an  example  of  one  form  of  the  rocks  of  this  class. 
We  have  seen  how  greatly  it  differs  from  such  rocks 
as  sandstone  or  chalk.  But  there  are  many  other 
varieties  of  crystalline  igneous  rock.  In  Fig.  25,  for 
example,  one  of  these  varieties  is  drawn.  It  is  a 


ROCKS.]  GEOLOGY. 


fragment  broken  from  a  current  of  lava  which  in  a 
molten  state  ran  down  the  side  of  a  volcano.  You 
observe  the  little  angular  crystals,  some  of  them 
black  and  large,  others  mere  white  specks  in  the 
general  mass  of  the  stone.  But  besides  the  crystals 


FIG.  26.— View  of  the  north  side  of  the  volcanic  cone  of  the  Island  of  Vol- 
cano, showing  a  stream  of  black  lava  which  has  not  flowed  down  to  the 
bottom  of  the  slope. 

you  notice  a  number  of  rounded  holes  or  cavities,  as 
if  little  water-worn  pebbles  had  fallen  out  of  the 
rock.  When  the  rock  was  still  melted  it  was  full  of 
imprisoned  steam  and  gas  which  were  constantly 
striving  to  escape  to  the  surface.  It  was  this  steam 


SCIENCE  PRIMERS. 


[IGNEOUS 


which  collected  into  little  bubbles  and  formed  the 
curious  set  of  holes  in  the  mass  of  the  still  molten 
rock.  In  the  same  way  the  holes  which  you  often 
see  in  the  heart  of  a  loaf  of  bread  were  formed  by 
the  struggles  of  the  steam  to  escape  from  the  dough 
as  it  was  heated  in  the  oven. 

1 68.  All  lava  belongs  to  this  class  of  rocks.  One 
or  two  pictures  may  serve  to  show  you  some  of  the 
more  simple  and  striking  features  of  these  lava 


FIG.  27. — View  of  Lava-stream  issuing  from  one  of  the  extinct  volcanic  cones 
of  Auvergne,  in  Central  France.     (Scrope.j 

masses.  In  Fig.  26,  a  drawing  is  given  of  part  of 
the  Island  of  Volcano,  in  the  Mediterranean,  in  which 
you  will  see  how  the  lava  has  risen  up  the  inside  or 
throat  of  the  volcanic  hill  to  the  edge  of  the  crater 
(see  Physical  Geogeraphy  Primer,  Art.  256),  and  run 
down  the  outside  of  the  slope.  When  that  took 
place  the  lava  was  of  course  thoroughly  molten 
like  liquid  iron,  and  hardened  as  it  cooled  in 
moving.  You  observe  that  it  has  not  been  able  to 


ROCKS.]  GEOLOGY.  79 

reach  the  bottom  of  the  hill.  It  was  in  truth  a 
very  small  stream,  cooling  and  hardening  before  it 
got  even  down  to  the  foot  of  the  slope.  But  look 
now  at  Fig.  27.  You  there  perceive  that  a  much 
more  copious  stream  of  lava  has  issued,  that  one 
side  of  the  volcanic  cone  has  been  broken  down  so 
as  actually  to  let  you  see  into  the  crater,  and  that 
the  lava  has  burst  out,  and  poured  down  the  sloping 
ground.  Thus,  each  outburst  of  lava  is  the  escape 
of  a  river  of  molten  rock  from  the  top  or  the  side 
of  "a  volcano.  Like  an  ordinary  river  of  water,  it 
of  course  sweeps  into  the  readiest  hollow  or  valley 
it  can  reach,  so  that  round  an  active  volcano  the 
valleys  often  get  quite  filled  up  and  buried  under 
the  vast  sheets  of  lava  which  are  poured  out.  Like 
rivers,  too,  the  streams  of  lava  vary  greatly  in  size.  In 
Fig.  26,  you  see  one  which  was  too  feeble  to  reach  the 
base  of  the  hill,  but  in  the  famous  eruption  of  Skaptar 
Jokul,  Iceland,  in  the -year  1783,  two  enormous 
streams  were  poured  out,  one  of  them  flowing  to  a 
distance  of  forty-five,  and  the  other  of  forty  miles. 
They  ranged  from  less  than  seven  to  twelve  or  fifteen 
miles  in  breadth,  with  a  thickness  of  a  hundred,  and 
sometimes  in  confined  valleys  even  six  hundred  feet. 

169.  If  you  paid  a  visit  to  any  ordinary  stream  of 
lava  after  it  had  come  to  rest  and  cooled,  you  would  find 
its  surface  to  be  an  irregular  accumulation  of  rough 
black  or  dark  brown  fragments  very  like  the  <;  slags  " 
or  "clinkers"  from  a  furnace.  Down  below  this 
rough  surface  the  rock  is  more  compact,  usually  dark 
in  colour,  containing  various  crystals  scattered  through 
its  mass,  and  often  full  of  holes,  as  shown  in  Fig. 
25.  In  some  cases  the  lava  as  it  grew  solid  has 


8o  SCIENCE  PRIMERS.  [IGNEOUS 

assumed  a  curious  and  beautiful  internal  arrange- 
ment into  columns.  The  pillars  of  Fingal's  Cave 
in  Staffa,  and  of  the  Giant's  Causeway  in  Antrim, 
have  been  produced  in  this  way.  At  both  of  these 
places  the  rock  was  once  a  molten  lava.  As  it  grew 
cold  and  solid  it  contracted,  and  in  so  doing  became 
divided  into  these  regular  columns.  You  might  imitate 


of  the  Island  of  Staffa,  with  Fingal  s  Cave. 


this  arrangement  by  putting  starch  in  warm  water, 
stirring  it  well  round,  and  then  letting  it  stand.  By 
degrees  you  would  observe  that  as  the  starch  grows 
solid  it  assumes  an  internal  columnar  arrangement, 
not  unlike  the  basalt. 

170.  Next  let  us  see  where  rocks  of  this  kind  are  to 
be  met  with.    Of  course  you  would  expect  to  find  them 


ROCKS.]  GEOLOGY.  Si 

round  the  flanks  of  an  active  volcano.  And  indeed, 
at  most  volcanos,  as  Vesuvius  or  Etna,  or  those  of 
Iceland,  they  abound.  But  you  would  trace  them  too 
round  volcanos  no  longer  in  activity,  as  for  example 
in  that  part  of  Central  France  where  the  extinct 
volcanos  occur  of  which  one  is  drawn  in  Fig.  27. 
And  as  you  travelled  over  the  world,  you  would 
recognize  them  in  hundreds  of  places  where  no  volcanic 
eruption  has  ever  been  known  since  human  history 
began.  In  fact  they  would  be  the  witnesses  to  you 
of  where  old  volcanos  had  once  been  at  work.  In 
this  way  by  learning  how  to  detect  these  forms  of 
ancient  lava,  you  would  be  able  to  prove  that  there 
had  been  volcanos  in  very  far  distant  times  in  dis- 
tricts where  there  are  now  busy  cities  or  fertile  fields. 

171.  For  example,  though  no  active  volcanos  exist 
in  Great  Britain  at  the  present  day,  they  can  be  shown 
to  have  often  broken  out  here  in  olden  times,  long 
before  man  appeared  upon  the  earth.     Some  of  the 
oldest  traces  of  volcanic  action  are  to  be  met  with 
in  North   Wales,  where  not  a  few  of  the  lava-beds 
form  prominent  features  in  the  scenery  of  that  rugged 
district.       Much   younger  are  the   sheets  of  various 
ancient   lavas   which   stretch   across    the    middle    of 
Scotland,  and  there  compose  most  of  the  hills.     But 
the  latest  of  the   British  volcanos  were  those  which 
ranged  in  a  long  line  from  Antrim  in  Ireland,  through 
the  Western   Islands,  and  away  north  by  the  Faroe 
Islands  into  Iceland.     The  vast  terraced  mountains 
of  Antrim,  Mull,  Skye,  and  Faroe  have  been  built  up 
of  piles  of  lava-sheets. 

172.  But  there  are  other  crystalline  igneous  rocks 
besides  those  which  come  to  the  surface  and  flow  out 


82  SCIENCE  PRIMERS.  [IGNEOUS 

there  as  molten  lava.  Granite,  for  example,  which  we 
have  already  examined  (Art.  26),  is  an  admirable  illus- 
tration of  the  crystalline  character.  But  instead  of 
coming  to  the  surface  to  cool  there,  granite  appears  to 
have  crystallized  and  cooled  deep  down  beneath  great 
masses  of  other  rocks.  Yet  it  now  forms  bare,  naked, 
lofty  mountains.  Many  of  the  Scottish  Highland  hills 
for  example,  as  Ben  Nevis,  Ben  Macdui,  and  Cairn 
Gorm,  consist  of  granite.  It  rises  high,  too,  in  the 
centre  of  the  chain  of  the  Alps.  Granite  often  sends 
out  veins  into  the  rocks  which  lie  above  and  around 
it.  It  could  not  have  done  this  unless  it  had  been  in 
a  fluid  or  pasty  state. 

173.  But  you  may  ask,  if  granite  has  not  crystallized 
at  the  surface,  but  under  masses  of  other  rocks,  how 
does  it  come  to  be  at  the  surface  now,  and  not  there 
only,  but  even  forming  the  crests  of  bare  lofty  moun- 
tains ?     This  question  is  not  quite  so  easy  to  dispose 
of,  but  you  will  probably  be  able  to  see  how  it  is  to 
be  answered  after  you  have  come  to  that  portion  of 
these  Lessons  which  treats  of  what  is  called  the  Crust 
of  the  Earth.     (See  Art.  239.) 

174.  2.   Fragmental    Igneous    Rocks.     The 
piece  of  stone  represented  in  Fig.  29  is  a  fragment 
from    a   bed  of  consolidated   volcanic   ashes.      You 
notice  that  it  is  made  up  of  irregular  and  angular  frag- 
ments.     These  are  little  bits  of  lava  and  other  rocks 
which  have  been  blown  into  the  air  by  the  discharges 
of  the  volcano.     You  observe  too  that  when  they  fell 
to  the  ground  and  accumulated  above  each  other  there 
they  took  a  stratified  form.    That  layer  of  coarse  frag- 
ments at  the  bottom  points  to  a  shower  of  coarservol- 
canic  ashes,  while  the  layers  of  smaller  fragments  above 


ROCKS.] 


GEOLOGY. 


show  how  showers  of  finer  dust  afterwards  fell  through 
the  air.  Now  this  is  the  kind  of  material  under  which 
the  old  Roman  city  of  Pompeii  was  buried  (Physical 
Geography  Primer,  Art.  259).  It  fell  upon  the  streets 
and  houses  and  gradually  covered  them  up  as  the 
eruption  of  the  neighbouring  volcano  continued. 
And  at  this  day  when  they  excavate  the  ruins,  the 
workmen  find  the  streets  and  chambers  all  choked  up 
with  layers  of  coarser  and  finer  volcanic  ash  and 
dust  arranged  just  as  you  see  in  Fig.  29. 


FIG.  29. — Piece  of  Volcanic  Tuff— a  rock  formed  of  consolidated  Volcanic 
ashes. 

175.  Of  course  if  the  volcanic  ashes  fell  over  the 
sea  or  a  lake  they  would  settle  down  beneath  the 
water  and  form  deposits  there.  They  might  cover 
up  and  preserve,  too,  the  remains  of  any  plants  or 
animals  which  might  be  lying  on  the  bottom  at  the 
time  of  the  eruption.  This  has  often  happened  in 
past  times.  In  the  mountain  of  Snowdon  in  Wales, 
for  example,  many  hundreds  of  feet  of  such  consoli- 
dated volcanic  dust  still  exist,  and  on  examining  this 
material  you  may  still  pick  out  shells  and  other  marine 
organisms  which  show  that  the  volcanic  materials 
fell  into  the  sea.  Again  in  Scotland  many  beds  of 


8a  SCIENCE  PRIMERS.  [IGNEOUS 

similar  nature  are  found  lying  among  seams  of  coal. 
These  masses  of  consolidated  volcanic  dust  and 
stones  are  known  by  the  name  of  TufI 

II.  Where  Igneous  Rocks  come  from. 

176.  If  I  ask  you  from  what  source  the   Igneous 
Rocks  have  been  derived,   you  will  reply  that  they 
have  come  up  from  the  intensely  hot  regions  within 
the  earth.     In  the  Physical  Geography  Primer  (Arts. 
252 — 265)  some  account  is  given  of  the  inside  of  the 
earth,  and  of  the  proofs  that  it  has  a  high  temperature. 
I  need  not  remind  you  what  a  very  little  part  of  the 
outer  portion  of  our  planet  we  can  actually  see,  even 
from  the  top  of  the  highest  mountain  to  the  bottom  of 
the  deepest  mine.    Let  us  go  over  in  this  Lesson  a  little 
more  in  detail  the  evidence  for  the  great  heat  of  the 
earth's  interior,  and  the  connection  between  that  heat 
and  certain  movements  and  changes  at  the  surface. 

177.  Deep  Borings  and  Mines.     If  you  were 
taken  down  to  the  bottom  of  a  deep  mine  in  England 
you  would  find  the  temperature  much  warmer  there 
than  near  the  surface,  and  a  similar  increase  of  heat 
would  meet  you  in  the  deep  mines  of  every  country 
in  the  world.     You  would  soon  discover,  too,  that  on 
the  whole  the  deeper  the  mine  the  greater  the  warmth 
would  be.     In  the  same  way  were  you  to  bore  a  deep 
narrow  hole  into  the  earth  for  several  hundreds  of 
feet  and  let  a  thermometer  down  to  the  bottom,  you 
would  find  that  the  mercury  would  rise  in  the  tube. 

178.  Experiments  of  this  kind  have  been  made  all 
over  the  globe,  with  the  result  of  showing  that  after  we 
get  down  for  a  short  and  variable  distance  below  the 
surface,  we  reach  a  temperature  which  remains  the 


ROCKS.] 


GEOLOGY. 


same  all  the  year,  and  that  underneath  that  limit  the 
temperature  rises  about  i°  Fahrenheit  for  every  fifty 
or  sixty  feet  of  descent.  If  this  rate  of  increase  con- 
tinues we  should  get  uncomfortably  hot  before  having 
descended  very  far.  For  instance,  at  a  depth  of 
about  two  miles  water  would  be  at  its  boiling-point, 


of  Hot  Springs  or  Geysers,  Iceland. 


and  at  depths  of  twenty-five  or  thirty  miles,  the  metals 
would  have  the  same  temperatures  as  those  at  which 
they  respectively  melt  on  the  surface  of  the  earth. 
It  is  clear  from  this  kind  of  evidence  that  the  inside  of 
our  planet  must  be  in  an  intensely  heated  condition. 


86  SCIENCE  PRIMERS.  [IGNEOUS 

177.  Proofs  of  another  kind  lead  to  the  same  con- 
clusion. The  city  of  Bath  has  long  been  famous  for 
its  wells.  Now  these  come  out  of  the  earth  at  a  tem- 
perature of  120°  Fahr.,  that  is,  rather  hotter  than  the 
water  is  usually  made  in  a  hot  bath.  And  this  warm 
water  has  been  rising  to  the  surface  and  flowing  to  the 
sea  ever  since  the  Romans  were  in  this  country,  and 
probably  long  before  that.  In  many  other  parts  of 


FIG.  31. — Vesuvius,  as  it  appeared  at  the  beginning  of  the  Christian  Era, 
when  it  was  a  dormant  volcano. 


the  world  similar  Hot  Springs  occur.  Iceland,  for 
example,  furnishes  some  remarkable  examples  called 
Geysers,  where  at  intervals  the  boiling  water  and  steam 
rush  out  with  a  great  noise,  and  rise  high  into  the 
air  (Fig.  30).  To  keep  up  such  hot  springs  in  every 
quarter  of  the  globe  there  must  assuredly  be  great 
stores  of  heat  within  the  earth. 

1 80.  Neither   the  heat  of  deep  mines   nor  of  hot 
springs  affords  such  an  impressive  lesson  as  to  the 


ROCKS.] 


GEOLOGY. 


earth's  internal  high  temperature  as  is  furnished  by 
Volcanos.  The  hot  vapours  and  steam  which  rise 
from  the  craters  of  volcanos,  the  torrents  of  hot  water 
which  sometimes  issue  from  their  sides,  the  streams 
of  molten  lava  which  break  out  and  roll  far  down  the 
slopes  of  a  volcanic  mountain,  burning  up  and  burying 
trees,  fields,  gardens,  and  villages — are  all  tokens  of 


FIG.  32. — Vesuvius  as  it  appears  at  the  present  time — an  active  volcano. 

the  intense  heat  of  the  inside  of  the  earth  from  which 
they  come. 

1 8 1.  At  the  present  time  there  are,  it  is  said,  about 
270  volcanos  either  constantly  or  at  intervals  throwing 
out  steam,  hot  ashes,  and  lava,  in  different  parts  of  the 
globe.  You  will  comprehend  how  widely  they  are 
distributed  if  you  again  take  a  map  of  the  world  and 
note  upon  it  the  lines  of  active  volcanos  (see  Phy- 
sical Geography  Primer,  Art.  260).  First  of  all,  down 
the  whole  line  of  the  mountains  which  range  along 
the  western  margin  of  the  American  continent,  vol- 


88  SCIENCE  PRIMERS.  [IGNEOUS 

canos  are  numerous,  some  of  them  of  vast  height 
like  Cotopaxi  (18,877  feet).  From  the  northern  ex- 
tremity of  America  they  extend,  by  way  of  the  Aleutian 
Islands  and  Japan,  to  the  Malay  Archipelago,  where 
in  Java  they  greatly  abound.  From  that  point  they 
may  be  traced  at  wide  intervals  into  New  Zealand  on 
the  one  hand,  and  on  the  other  through  the  centre  of 
Asia  by  way  of  the  Red  Sea  and  the  Mediterranean, 
up  to  Iceland  and  down  to  the  Azores,  and  thence 
across  to  the  West  Indies  and  the  centre  of  America. 
Even  among  the  perpetual  snows  of  the  South 
Polar  regions  they  have  been  met  with,  and  also 
far  within  the  Arctic  Circle  at  the  Island  of  Jan 
Mayen. 

182.  But  besides  these  volcanos  which  are  still  active, 
many  others  occur  from  which  no  eruptions  have  ever 
been  seen  to  take  place,  and  which  are  therefore  called 
dormant  or  extinct  (see  Figs.  27  and  31).  If  you 
were  to  put  down  upon  a  map  the  position  of  every 
volcano  which  either  now  or  at  some  past  time  has 
given  out  hot  gases,  steam,  ashes,  or  lava,  you  would 
probably  find  very  few  large  areas  of  the  earth's  surface 
in  which  no  trace  of  volcanic  action  can  be  found. 
Britain,  for  instance,  is  now  wholly  free  from  any  vol- 
canic disturbance,  and  yet,  as  already  pointed  out 
(Art.  171),  you  would  need  to  mark  many  places  on 
the  map  of  this  country  as  having  once  been  the  scene 
of  long-continued  volcanic  eruptions.  You  would 
have  to  put  some  dots  upon  the  map  round  Exeter 
to  mark  the  position  of  some  ancient  volcanos  ;  a 
good  many  in  Wales,  some  in  Derbyshire,  and  others 
in  Cumberland.  You  would  have  to  cover  almost  all 
the  centre  of  Scotland  with  such  dots,  for  that  region 


ROCKS.]  GEOLOGY. 


is   full    of  volcanic   rocks,    and    in  Ireland  too  there 
would  be  a  good  many  scattered  marks. 

183.  In  this  way  you  would  come  to  see  how  uni- 
versal volcanic  action  has*  been,  on  the   whole,  over 
the  globe,  and  therefore  how  powerfully  and  general. y 
the  heat  of  the  interior  has  manifested  itself  at  the 
surface. 

184.  But  in  igneous  rocks  you  do  not  see  the  only 
evidence  of  how  the  internal  heat  affects  the  surface 
of  the  earth.     There  can  be  little  doubt  that  Earth- 
quakes (Physical  Geography  Primer,  Art.  262)  must 
be  mainly  due  to  commotions  which  take  their  origin 
from  the  effects  of  this  heat. 

185.  Perhaps  you  will  ask,  why,  since  the  inside  of  the 
planet  is  so  hot,  does  it  not  melt  the  outside,  or  at  least 
why  is  the  outside  not  warmer  ?     There  can   be  no 
doubt  that,  at  one  time  many  millions  of  years  ago,  the 
globe  was  immensely  hotter  than  it  is  now.     In  fact  it 
then  resembled  our  burning  sun,  of  which  it  once  pro- 
bably formed  a  part,  and  from  which  it  and  the  other 
planets  were  one  by  one  detached.     During  the  vast 
interval  which  has  passed  away  since  then  it  has  been 
gradually  cooling,  and  thus  the  heat  in  the  inside  is 
only  the  remains  of  that  fierce  heat  which  once  marked 
the  whole  planet.     The  outer  parts  have  cooled  and 
become  solid,  but  they  are  bad  conductors  of  heat, 
and  allow  the  heat  from  the  inside  to  pass  away  into 
space  only  with  extreme   slowness  (Physics   Primer, 
Arts.  64,  65).     Hence,  in  spite  of  the  high  tempera- 
ture of  the  interior,  we  are  not  sensible  that  it  warms 
the  outer  surface  of  the  earth. 

1 86.  To  illustrate  this  point  suppose  that  you  could 
see  a  volcano   in   the  very  act   of  pouring  a  great 


90  SC1EXCE  PRIMERS.  [THE  CRUST 

stream  of  molten  lava  down  its  slopes.  At  first  the 
torrent  would  be  at  a  white  heat,  glowing  so  fiercely 
that  you  could  hardly  keep  your  eyes  upon  it.  But 
a  few  yards  below  the  point  from  which  it  emerged 
it  would  begin  to  assume  a  reddish  hue,  which  would 
get  duller  and  darker,  just  as  a  live  coal  does  when 
'it  falls  from  the  grate  upon  the  hearth,  and  the 
surface  of  the  lava  would  at  the  same  time  get  cool 
and  solid  so  quickly  that  in  a  few  days  you  might 
stand  upon  it,  even  though  it  was  still  red  hot  only  a 
foot  or  two  below.  You  might  come  back  to  it  a 
dozen  of  years  afterwards.  Its  surface  would  be  per- 
fectly cold — a  mere  rough  sea  of  black  bristling  lumps 
of  rock — and  yet  down  in  the  depths  of  the  mass  the 
rock  would  be  still  hot,  and  you  might  even  meet  with 
cracks  from  which  the  heat  escapes  along  with  wreaths 
of  steam,  and  where  therefore  you  could  not  hold  your 
hand  without  having  it  burnt.  Now  if  a  mere  river  of 
lava  takes  so  long  to  cool  down  to  its  centre,  you  can 
realize  perhaps  why  it  is  that  the  huge  mass  of  our 
globe  should  still  be  intensely  hot  inside  even  though 
its  outer  portions  have  been  solid  and  cool  for  long 
ages. 

187.  You  are  already  familiar  with  the  fact  that 
bodies  expand  when  they  are  heated,  and  contract  as 
they  cool  (Physics  Primer,  Art.  49).  When  the  earth 
was  vastly  hotter  than  now  it  must  also  have  filled 
more  space.  While  cooling  it  has  been  contracting. 
As  it  is  still  cooling  it  must  be  still  contracting,  but  so 
slowly  that  on  the  whole  we  are  not  sensible  of  the 
process.  But  some  of  the  effects  are  visible  enough 
among  the  rocks.  The  contraction  could  not  fail  to 
cause  an  enormous  pressure  or  strain  on  the  outer 


OF   THE   EARTH.]  GEOLOGY.  91 

parts,  which,  since  they  are  made  of  such  very  various 
materials — Sedimentary,  Organic,  and  Igneous  Rocks 
— would  yield  to  the  stress  more  in  some  places  than 
in  others.  And  thus,  somewhat  like  the  skin  of  a 
dried  and  shrivelled  apple,  the  surface  of  the  globe 
would  be  ridged  up  in  one  region,  or  would  sink  down 
in  another,  besides  being  squeezed  and  broken.  What 
evidence  we  have  for  all  this  will  be  told  in  the  next 
Lessons. 

THE    CRUST   OF   THE    EARTH. 

I.  Proofs  that  parts  of  the  Crust  have  been 
pushed  up. 

1 88.  We  have  now  completed  the  first  part  of  the  task 
which  was  proposed  in  an  earlier  Lesson  (Art.   7) — 
to  find  out  what  the  materials  are  of  which  the  great 
stone  floor  of   the  earth  is  made.     We  have  learnt 
something  about  three  great  classes  of  rocks   which 
form. that  floor — how  they  were  made,  and  where  they 
are -to  be  seen.     But  while  learning  these  facts  about 
the  earth,  we  have  seen   that   the  rocks   are   not  a 
mere  thin  covering  like  a  wooden  floor  below  which 
we  should  come  to  something  quite  different.      We 
cannot  get  down  beneath  the  rocks.     Deep  as  the 
deepest  mine  the  same  kind  of  rocks  may  be  found 
which  elsewhere  exist  at  the  surface.     It  is  always 
through  rock  of  some  kind  that  we  must  descend  as 
far  as  we  can  penetrate  into  the  bowels  of  the  earth. 

189.  This  solid  rocky  outer  part  of  the  earth  on  which 
we  live,  into  which  men  sink  mines  and  out  of  which 
springs  arise,  is  called  the   Earth's  Crust.      This 


92  SCIENCE  PRIMERS.  [THE  CRUST 

name  came  into  use  when  people  supposed  that  all 
the  inside  of  the  planet  was  an  intensely  hot  liquid 
mass  with  a  cool  and  comparatively  thin  crust  outside. 
A  great  deal  of  dispute  has  arisen  as  to  whether  the 
main  mass  of  the  inside  of  the  earth  is  liquid  or 
solid,  but  those  who  dispute,  whatever  their  view  may 
be,  agree  to  use  this  phrase  the  Earth's  Crust  as 
meaning  that  part  of  the  earth  which  men  can  observe 
from  the  top  of  the  highest  mountain  to  as  far  below 
the  deepest  mine  as  they  can  reasonably  infer  what  the 
rocks  must  be. 

190.  The  rocks  of  which  this  crust  consists  belong 
mostly  to  the  Sedimentary  series,  a  large  number  to  the 
Organic  series,  and  a  smaller,  but  still  considerable 
proportion,    to    the  Igneous    series.      In   Britain,   for 
example,  if  we  could   put  all  the  different    series  of 
sedimentary  and  organic  rocks   together,    one  above 
another,  in  the  order  in  which  they  were  deposited, 
they  would  form  a  mass  at  least  ten  or  twelve  miles 
thick.     Out  of  such  materials  the  solid  earth  is  built 
up  as  far  down  into  its  depths  as  man  has  been,  able 
to  descend. 

191.  But  from   what   has  been  stated  in   previous 
Lessons  it  is  clear  that  many  of  these  rocks  are  not 
now  in  their  original  positions.     Our  quarry,  for  ex- 
ample (Art.   119),  told  us  how  the  rocks  in  which  it 
lay  had  once  formed  a  part  of  the  sea-bottom.     Then 
again  the  coal-seams,  buried  so  deep  in  the  earth,  were 
once  verdant  forests  or  jungles  at  the  surface   (Art. 
139).    How  could  a  sea-floor  become  dry  land,  and  how 
could  a  forest  on  the  surface  of  the  land  come  to  be 
covered  by  hundreds  of  feet  of  solid  stone  ? 

192.  Let  us  begin  by  considering  how  it  is  that  a 


OF    THE    EARTH.] 


GEOLOGY. 


93 


portion  of  the  floor  of  the  sea  can  be  changed  into  good 
dry  land.  And  in  order  to  follow  the  change  as  clearly 
as  possible  we  shall  choose  one  of  the  simplest  ex- 
amples, and  one  moreover  which  many  of  us  may 
have  the  opportunity  of  verifying  for  ourselves. 

193.  Round  the  coast-line  of  some  parts  of  the  British 
Islands  there  runs  a  low  flat  terrace  bounded  by  the 
sea  on  the  one  hand  and  by  a  cliff  or  inland  slope 


FIG.  33.— View  of  a  R; 


on  the  other.  Seaport  towns  have  been  built  upon 
this  terrace,  such  for  instance  as  parts  of  Glasgow, 
Greenock,  and  Leith.  It  is  so  level  that  roads  run 
along  its  surface  for  miles  among  cornfields,  meadows, 
and  villages.  You  may  gather  some  notion  of  its 
general  appearance  from  Fig.  33,  which  shows  how  flat 
it  is  and  how  little  elevated  above  the  sea  at  its 


94  SCIENCE  PRIMERS.  [THE  CRUST 

outer  edge.  Along  its  inner  margin  there  often  rises 
a  line  of  cliff  pierced  with  caves,  as  represented  in  the 
drawing.  If  you  were  standing  on  some  part  of  this 
terrace  and  looking  along  its  level  surface  as  it  winds 
in  and  out  against  the  cliffs  and  slopes  of  the  land, 
would  not  the  idea  of  an  old  coast-line  at  once 
suggest  itself  to  your  mind  ?  You  can  without  diffi- 
culty picture  the  sea  covering  that  terrace  and  beating 
against  the  base  of  those  cliffs  and  slopes. 

194.  Could  you  prove  this  fancy  of  yours  to  be  any- 
thing more  than  a  fancy  ?  Let  us  see.  Cross  to  the 
inner  margin  of  the  terrace  and  consider  attentively  the 
line  of  caves  you  find  there.  How  did  these  excava- 
tions come  to  be  drilled  into  the  solid  rock  all  along  the 
same  line  and  exactly  at  the  same  level,  so  that  the 
floor  of  each  of  them  should  just  open  upon  the  flat 
terrace?  Suppose  that  you  visit  one  of  these  caves. 
Festoons  of  ivy  and  honeysuckle  hang  perhaps  in 
tangled  luxuriance  about  its  mouth,  and  you  may  have 
to  force  your  way  through  a  brushwood  of  strong 
briars.  But  you  gain  at  last  the  floor  of  the  cave, 
which  you  find  to  be  roughened  with  rounded,  water- 
worn  stones.  The  roof  is  partly  hung  with  ferns, 
mosses,  and  liverworts,  and  the  sides  too  have  their 
drapery  of  green.  But  the  bare  rock  appears  abun- 
dantly, and  you  notice  that  it  has  been  rubbed  smooth, 
and  has  the  same  water-worn  character  as  the  stones 
under  your  feet.  Now  go  outside  and  look  at  the 
bare  rocks  of  the  cliff  above  ;  you  see  how  rough  and 
sharp-edged  they  are,  as  from  time  to  time  they  split 
up  under  the  influence  of  the  weather.  The  walls  of 
the  cave  have  been  ground  smooth  from  one  cause, 
the  face  of  the  cliff  has  been  made  rough  from  another. 


OF  THE  EARTH.]  GEOLOGY.  95 

195.  The  explanation  of  this  difference  will  be  appa- 
rent if  you  remember  what  takes  place  where  a  sea- 
cliff  of  hard  rock  has  its  base  washed  by  the  waves 
(Arts.  73—75).  You  have  seen  how  the  rocks,  wherever 
the  waves  can  reach  them,  are  worn  smooth  by  the 
ceaseless  grinding  of  the  gravel  and  stones  to  and  fro. 
And  every  cave  into  and  out  of  which  the  waves  drive 
the  gravel  is  ground  down  in  the  same  way.  A  single 
forenoon  spent  on  such  a  coast-line  gives  you  a  lesson 
you  can  never  forget  as  to  the  way  in  which  rocks  have 
their  surfaces  smoothed  by  the  waves.  But  all  which 
lies  above  the  reach  of  the  breakers  comes  under  the 
influence  of  other  forces.  Rain,  frost,  and  springs  com- 


P resent  ocach     /      . .  .  «   — — •?-^~^-^-~r~T~^~- 


FIG.  34.  -  Section  of  a  Raised  Beach. 

bine  to  make  the  cliff  crumble  down,  and  fragments  to 
split  off  from  its  face  so  as  to  give  that  rough  angular 
appearance  which  contrasts  so  well  with  the  water- 
worn  rocks  below. 

1 96.  After  having  observed  in  this  way  what  is  taking 
place  now  along  a  sea-cliff,  you  can  hardly  doubt  that 
the  line  of  cliff  which  rises  from  the  inner  margin  of  our 
terrace  was  once  a  sea-cliff  too,  with  the  waves  beating 
against  its  base  and  boring  that  line  of  caves  there, 
as  they  are  still  doing  elsewhere.  The  line  of  that 
cliff  thus  becomes  in  your  mind  the  line  of  a  former 
sea- shore. 


96  SCIENCE  PRIMERS.  [THE  CRUST 

197.  But  further  proofs  of  the  former  presence  of  the 
sea  present  themselves  if  you  put  questions  to  the  terrace 
itself.     Dig  beneath  the  surface  of  that  terrace  any- 
where, and  what  do  you  find  it  to  consist  of?     Sand 
and  gravel,  sometimes  with  abundance  of  shells.     Look 
at  the  outer  margin  of  the  terrace  where  the  sea  \-> 
gradually  cutting  it  away,  and  you  find  there  that  the 
sand  and  gravel  are  laid  down  in  layers,  just  as  they 
are  on  the  beach  below,  and  that  the  shells  belong  to 
the  common  kinds  which  are  washed  up  by  every  tide 
upon  the  sands.     You  discover  that  in  truth  the  terrace 
is  simply  an  old  beach,  and  that  the  sea  must  have 
laid  down  the  materials  of  the  terrace  when  it  was 
scooping  out  the  caves  at  the  foot  of  the  cliff.     Thus 
the  terrace  and  the  caves  combine  to  prove  a  change 
in  the  coast-line. 

198.  By  measuring  the  height  of  the  floor  of  the 
caves  and  the  height  of  the  terrace  above  the  present 
high-water  mark,  you  would  ascertain  the  difference 
of  level  between  the  present  beach  and  the  old  one. 
Let  us  suppose  that  this  difference  is  in  the  present 
case  twenty  feet ;  it  is  plain  that  the  land  must  have 
risen,  or  the  sea  must  have  sunk,  to  the  extent  of 
twenty  feet. 

199.  When  you  watch  the  restlessness  of  the  sea,  with 
its  ebbing  and  flowing  tides,  its  waves  and  currents, 
and  then  when  you  contrast  with  this  ceaseless  motion 
the  calm  steadfastness  of  the  land,  you  may  naturally 
suppose  that,  in  any  changes  of  the  relative  position 
of  land  and  sea,  it  is  much  more  likely  that  the  sea 
should  have  shifted  its  place  than  that  any  alteration 
should  have  happened  to  the  land.     But  reflect  for  a 
moment  on  what  would  be  involved  in  a  change  in 


OF  THE  EARTH.]  GEOLOGY,  97 

the  sea-level  at  any  place.  If  I  deepen  the  bottom 
of  one  end  of  a  pond,  does  the  level  of  the  water  fall 
just  over  that  part  where  I  have  been  at  work? 
Assuredly  not :  the  level  of  the  whole  pond  is  lowered. 
Tn  the  same  way,  if  I  empty  a  quantity  of  stones  and 
earth  so  as  to  make  one  part  of  the  pond  very  much 
shallower,  do  I  raise  the  level  of  the  water  only  over 
that  part  ?  By  no  means  :  the  influence  of  what  I 
have  been  doing  extends  through  every  part  of  the 
pond,  and  the  level  of  the  water  is  uniformly  raised 
over  the  whole. 

200.  Now  instead  of  the  pond,  think  of  the  great 
ocean,  which  is  only  an  enormously  large  continuous 
sheet  of  water.    You  see  that  an  alteration  of  its  level 
in  one  region  must  necessarily  extend  over  the  whole 
globe,  until  the  same  general  uniformity  of  level  is  re- 
stored.   If  the  sea  has  sunk  from  our  terrace  (Figs.  33, 
34)  to  the  extent  of  twenty  feet,  there  must  have  been 
at  the  same  time  a  general  lowering  of  the  sea-level 
all  over  the  world.     But  is  it  so?     How  would  you 
set  about  to  ascertain  this  point  ? 

201.  Plainly   if    the   terrace    has    been   left   by    a 
sinking  down  of  the  bed  of  the  sea,  you  should  meet 
with   a   corresponding    terrace    all    over   the    globe. 
But  you  would  not  require   to  travel  far  before  you 
ascertained  that  no    such  universal   terrace  is  to  be 
seen.     Even  around  the  coast  of  Britain  you  would 
find  enough  to  show  you  that  there  has  not  been  any 
general  subsidence  of  the  ocean.     Throughout  a  great 
part  of  the  margin  of  our  island  no  terrace  occurs  at 
all.     Only  in  certain  districts  is  such  a  terrace  to  be 
met  with,  and  its  height  is  not  always  the  same. 

202.  Sometimes  a  series  of  terraces  maybe  seen  rising 


98  SCIENCE  PRIMERS.  [THE  CRUST 

one  over  another,  each  marking  a  former  coast-line. 
In  the  north  of  Norway  they  occur  in  great  perfec 
tion  (Fig.  35),  up  to  heights  of  several  hundred  feet. 
They  look  perfectly  horizontal  to  the  eye,  yet  when 
they  are  measured  accurately  they  are  found  some- 
times to  rise  towards  the  upper  end  of  the  long  inlets 
on  the  sides  of  which  they  run,  so  that  a  terrace 
which  at  the  sea-ward  end  may  stand  at  a  height 
of  80  feet  above  the  sea,  is  as  much  as  90  or  100 


FIG.  35.— Terraces  [Raised  Beaches)  of  the  Alien  Fjord,  Norway. 


feet  at  the  land-ward  end.  Now  such  a  difference 
of  level  in  a  short  distance  proves  that  something 
else  must  have  taken  place  than  a  mere  subsidence  of 
the  sea,  for  had  that  been  the  cause  of  the  terraces 
being  left,  they  should  all  have  been  as  horizontal  as 
the  surface  of  the  sea  itself,  and  at  least  traces  of 
them  should  have  been  found  at  corresponding  heights 
in  our  own  country  and  all  over  the  world. 


OF  THE  EARTH.]  GEOLOGY.  99 

203.  Strange  as  it  may  seem  to  you,  it  is  nevertheless 
true,  that  it  is  the  land  which  rises,  not  the  sea 
which  sinks.     If  that  be  the  case,  it  is  easy  to  see  how 
there  should  be  terraces  in  some  countries  and  not  in 
others,   and   how  the   same    terrace    should   vary  in 
height  at  different  parts  of  its  course.     For  the  land 
may  have  been  pushed  up  at  one  place  and  not  in 
others,  and  more  at  one  place  than  in  another.     The 
old  sea-terrace  (Fig.  33)  is  called  a  Raised  Beach, 
because  it  consists  of  gravel,  sand,  and  other  beach 
deposits,  which  have  been  raised  above  the  level  of 
the  sea.     Every  such  raised  beach  points  to  a  former 
sea-margin,  and  to  an  elevation  of    that  sea-margin 
into  dry  land.     Where  a  great  many  terraces  occur 
one  above  another,  as  in  Norway  (Fig.  35),  they  show 
us  how  the  land  has  been  raised  up  at  intervals  for  a 
long  period,  the  time  when   the  land  was   stationary 
between  two  upheavals  being  marked  by  a  terrace  or 
raised  beach.     Of  course   the   highest    terrace  must 
needs  be  the  oldest,  and  for  that  reason  is  often  less 
perfect  than  the  newer  ones,   having  suffered  more 
from   the   various   forces   such    as   rains,   frosts,   and 
streams,  which  are  so  busy  in  making  the  surface  of 
the  land  crumble  away.    (Physical  Geography  Primer. 
Art.  126.) 

204.  In  some   parts  of   the  world  we  can  detect 
the  ground  in  the  very  act  of  rising.      In  the  south- " 
east  of  Sweden,  for  example,  rocks  have  been  marked 
at  the  place  where  high-water  reached  them,  and  in 
the  course  of  years  have  been  found  to  be  consider- 
ably above  their  former  level.     From  observations  of 
this  kind  it  has  been  inferred  that  the  land  there  is 
rising  at  the  rate  of  about  two  or  three  feet  in  a  century. 


ioo  SCIENCE  PRIMERS.  [THE  CRUST 

This  appears  to  be  but  a  very  slow  movement,  too 
slow  to  be  appreciated,  except  by  careful  measure- 
ment ;  and  yet  if  it  were  to  go  on  for  another  thou- 
sand years,  what  is  now  the  beach  would  have  risen 
to  a  height  of  twenty  or  thirty  feet  above  the  sea- 
level. 

205.  You  see,  then,  that  the  upraising  of  the  bottom 
of  the  sea,  strange  as  it  may  seem  to  us,  is  not  entirely 
a  thing  of  the  past.     It  is  going  on  slowly  at  the  pre- 
sent time  in  several  parts  of  the  globe.      And  just  as 
the  coast  of  Sweden  is  rising  with  no  violence  or 
shock,  so  in  old  times  the  upraising  of  the  sea-bed 
into   dry  land   may  have   been   a   gentle   and   quiet 
process. 

206.  The  rocks  of  every  country  furnish  abundant 
evidence  that   the    sea-bottom   has   again  and  again 
been  elevated  into  land.     This  evidence  is  furnished 
as  you  now  know,  chiefly  by  the  remains  of   corals, 
star-fishes,  shells,  and  other  sea-creatures,  which  may 
be   traced   imbedded  in  the   rocks.     The   height   at 
which  these  remains  are  found  affords  us  some  idea 
of  the  extent  of   the  elevation.     The  shells  of  our 
raised  beach  (Art.  197)  indicated  a  rise  of  only  some 
twenty  feet.  But  if  you  found  such  sea-shells  at  a  height 
of  twenty  thousand  feet  they  would  prove  that  the  bed 
of  the  sea  had  been  elevated  at  least  to  that  extent 
(Art.  128).     By  this  kind  of  evidence  it  can  be  shown 
that  by  much  the  greater  part  of  the  dry  land  has  been 
raised,  piece  by  piece,  out  of  the  sea,  and  that  the 
movements  have   been  far  from  regular  or  uniform, 
seeing  that   some  parts  have    been  upheaved  to    a 
much  greater  height  than  others. 


OF   THE   EARTH.]  GEOLOGY. 


II.  Proofs  that  parts    of   the    Crust    have 
sunk  down. 

207.  We  have  now  traced  out  some  facts  which  show 
that  the  surface  of  the  globe  has  from  time  to  time 
been  pushed  up,  so  that  parts  of  the  sea-bottom  have 
become  dry  land.     But  other  movements  of  exactly 
an  opposite  kind  have  turned  parts  of  the  land  into 
the  bed  of  the  sea.     Let  us  follow  some '  of  the  evi- 
dence for  these  depressions,  and  take,  as  before,  our 
illustrations  from  places  which  are  easily  visited,  and 
in  our  own  country. 

208.  Along  some  parts  of  the  coast-line  of  Britain, 
as  for  example  on  the  coasts  of  Devon  and  Cornwall, 
and  on  that  of  the  Firth  of  Tay,  a  very  curious  and 
interesting  feature  occurs  between  high  and  low-water 
mark.     From  the  flat  sandy  surface  of  the  beach  a 
number  of  dark  stumps  may  be  seen  sticking  up,  which 
on  closer  examination  prove  to  be  the  lower  ends  of 


LOW  WATER  MARK 


FIG.  36. — Section  of  a  Submerged  Forest. 

trees.  Scraping  away  the  sand  of  the  beach  you  meet 
with  dark  loam  or  earth,  out  of  which  the  tree- 
stumps  protrude,  and  from  which  you  may  pick  up 
hazel  nuts,  leaves,  twigs,  and,  now  and  then  perhaps, 
the  wing-case  of  a  beetle,  or  the  bone  of  some  land 
animil.  As  you  trace  stump  after  stump  along  the 


102  SCIENCE  PRIMERS.  [THE  CRUST 

beach,  you  see  that  they  are  all  in  the  usual  upright 
posture  in  which  trees  grow.  The  dark  earth  in  which 
the  tree-roots  spread  is  clearly  an  ancient  soil,  in 
which  to  this  day  may  be  gathered  the  very  leaves, 
twigs,  and  nuts  which  fell  from  the  trees,  and  fragments 
of  the  insects  which  lived  amid  their  decaying  timber. 
These  stumps  on  the  beach  are  evidently  parts  of 
an  old  forest  or  wood. 

209.  But  could  the  trees  ever  have  grown  where  their 
remains  are  now  to  be  seen  ?     By  no  means.     The 
hazel,  birch,   alder,  and   oak,  of  which   the   stumps 
mostly  consist,  would  be  killed   if  their  roots   and 
trunks  were  to  be  permanently  submerged  in  the  sea. 
You  never  see  any  of  these  trees  growing  below  tide- 
mark  now,  and  you  cannot  suppose  ihat  they  ever  did 
so.     If  the  trees  on  the  beach  must  have  grown  where 
their  remains  still  exist,  and  if  they  could  not  have 
grown  up  in  the  sea,  then,  either  the  sea  must  have 
risen  up  so  as  to  cover  them,  or  the  land  must  have  sunk 
down  so  as  to  submerge  them.     But  we  have  already 
learnt  (Art.   203)  that  in  all  such  cases  of  change  of 
level  we  cannot  believe  that  the  sea  alters  its  level 
to  any  appreciable  extent,  so  that  we  must  conclude 
that  the  submergence  of  the  old  trees  has  been  due  to 
a  sinking  down   of   the  land.     These  Submerged 
Forests,  therefore,  are  to  be  regarded  as  evidence  of 
subsidence  of  the  earth's  surface,  just  as  the  raised 
beaches  are  taken  as  proofs  of  upheaval. 

210.  You  can   understand  that  it  must   be  more 
difficult  to  trace  evidence  of  ground  having  sunk  than 
of  its  having  risen  in  level.     Because  when  any  district 
has  gone  down  below  the   sea,  the  waves  gradually 
obliterate  all  trace  of  the  former  land  surface,  as  they 


OF   THE    EARTH.] 


GEOLOGY. 


103 


are  now  washing  away  the  submerged  forests  ;  while, 
on  the  other  hand,  when  the  bed  of  the  sea  is  turned 
into  dry  land  such  traces  as  raised  beaches,  and  old 
sea-worn  caves,  remain  to  mark  the  space  which  the 
salt  water  once  covered. 

211.  In  different  parts  of  the  globe  it  has  been  ob- 
served that  the  sea  appears  to  be  gradually  rising  upon 


FIG.  37. — Sectioi 


of  the  Strata  in  a  Coal-pit,     c,  Coal-s 
fracture  of  the  rocks. 


ams.    f,  a  fault  ot 


Jie  land.  In  reality  it  is  the  land  which  is  there 
sinking  below  the  sea.  For  example,  the  southern 
part  of  Greenland  for  several  hundred  miles  has  during 
the  last  few  centuries  been  slowly  subsiding,  so  that 
rocks  which  once  lay  above  the  limits  of  the  tides  are 
now  submerged,  and  the  dwelling-houses  of  the  inhabi- 
tants have  had  to  be  shifted  further  and  further  inland. 
10 


104  SCIENCE  PRIMERS.  [THE  CRUST 

212.  Other  proofs  of  the  same  fact  have  already 
been  referred  to  in  the  foregoing  Lessons.     The  beds 
of  coal,  for  example,  which  once  flourished  as  green 
forests  at  the  surface  are  now  found  buried  deep  within 
the  earth.     By  what  process  did  they  get  there  ?     Let 
us  return  for  a  while  to  the  Coal-pit  referred  to  in 
Art.  137. 

213.  In  many  parts  of  Britain  the  coal-pits  are  more 
than  a  thousand  feet  deep.     And  yet  down  at  the 
bottom  of  each  of  these  pits  lies  the  coal-seam. 'which 
we  have  found  to  be  a  buried  swamp  or  jungle.     If 
you  could  look  at  all  the  rocks  which  have  been  cut 
through  in  making  the  long  shaft  of  the  pit,  you  would 
usually  find  among  them  other  coal-seams  than  the 
•one  at  the  bottom.     In  fact,  several  seams  are  some- 
times worked  for  coal  at  different  levels  in  the  same 
pit.     You  will    understand    their    position    from   the 
section  in  Fig.  37,  which  shows  how  the  rocks  lie  one 
above   another  in   one  of  these    pits.      You   notice 
that   the  seam  down  to  which   the    shaft   has  been 
sunk  is  the  fifth  of  the  series,  but  it  is  chosen  in  the 
meantime,  probably   because    it  is  a  better  kind  of 
coal  than  the  other  four  seams  above  it,  and  therefore 
brings  more  money  in  the  market 

214.  In  such  a  section   as  that  in  Fig.  37,  which 
shows  only  what  may  be  met  with  in  any  coal-field. 
we  see  that  the  strange  revolution  whereby  a  green 
waving   forest   has    been   buried    underground    must 
have  happened  not  once  only,  but  many  times  ;  for 
every  separate  coal-seam  was  evidently  at   one  time 
a  verdant  plain,   open  to  the   sun,   and  bright  with 
many   a    graceful    tree   and    fern.      And    still   more, 
besides  the  evidence  of  the  coal-seams,  upright  stems 


OF   THE   EARTH.] 


GEOLOGY. 


105 


of  trees,  now  turned  into  stone,  are  sometimes  found 
standing  in  the  sandstones  and  shales,  in  the  very 
position  in  which  they  grew  with  their  roots  even 
yet  imbedded  in  the  ancient  soil.  (Fig.  38)'. 


FIG.  38— Section  of  a.  part  of  the  Cape  Breton  Coal-field,  showing  seven 
ancient  soils,  with  remains  of  as  many  forests.  (R.  Brown.)  a,  Sand- 
stones, b  znde,  Shales;  c.  Coal-seams  ;  d,  Underclays,  or  Soils. 

215.  The  lowest  strata  are  of  course  the  oldest  (see 
Art   122).      Hence   the  undermost   coal-seam   must 


io6  SCIENCE  PRIMERS.  [THE  CRUST 

have  been  buried  before  the  later  forests  could  spring 
up  on  its  site.  It  grew  probably  in  a  wide,  marshy 
plain,  which  when  the  ground  sank  down  became  a 
wide  sheet  of  water.  Sand  and  mud  were  carried 
into  this  water,  and  laid  down  upon  the  submerged 
forest.  These  sedimentary  deposits  may  now  be  traced 
in  the  beds  of  sandstone  and  shale  which  overlie  the 
coal-seam.  The  sand  and  mud  brought  into  the  wide 
and  shallow  sheet  of  water  might  in  the  end  fill  it 
up  so  that  at  length,  as  the  muddy  bottom  rose 
to  the  surface,  a  new  mass  of  vegetation  would  take 
root  and  form  as  luxuriant  a  growth  as  the  buried 
forest  had  done.  But  after  this  had  taken  place 
the  downward  movement  of  the  ground  again  showed 
itself,  for  this  second  forest  was  carried  beneath  the 
water  and  covered  with  renewed  accumulations  of 
sand  and  mud. 

216.  Hence  we  learn  that  our  coal-fields  were  formed 
in  regions  which  were  sinking,  and  that  the  downward 
movement  was  not  continuous,  but   went  on    at   in- 
tervals.    That  it  must  have  been  prolonged   through 
vast  periods  of   time  is  apparent  from    the  fact  that 
the  strata  of  the    coal-fields  are  many  thousands  of 
feet  thick,  and   must  hence  have  needed  long  ages 
for  their  formation. 

217.  Two  facts  are  now  very  clear  to  you  about  the 
crust  of  the  earth — ist,  it  has  often  been  pushed  out- 
wards, so  as  to  rise  above  the  level  of  the  sea ;  and 
2nd,   it  has  also  often  sunk  inwards   so  as   to  carry 
parts  of  the  land  deep  beneath  the  sea-level.     But  it 
could  not  undergo   these  movements  without   suffer- 
ing other  changes,  which  will  be  considered  in  the 
next  Lesson. 


OF  THE   EARTH.]  GEOLOGY.  JO? 


III.  Proofs  that  the  Rocks  of  the  Earth's 
Crust  have  been  tilted,  crumpled,  and 
broken. 

218.  If  you  think  about  the  movements  described  in 
the  two  previous  Lessons,  and  consider  how  often  the 
crust  of  the  earth  must  have  been  pushed  up  or  let 
down,  you  will  not  be  surprised  to  find  that  the  rocks 
have  not  only  been  shifted  up  or  down,  but  have  been 
crumpled  up  and  broken  across.     Hence  the  crust  of 
the  earth,  instead  of  being  made  of  regular  layers 
one  above  another,  like  the  coats  of  an  onion,  has 
been  so  squeezed  and   fractured,  that  in  many  cases 
the   bottom   or    oldest  rocks  have  been  pushed    up 
far  above  the    newest.     Let  us    clearly  understand 
how  this  statement  can  be  made  out;  and  for  that 
purpose  we  shall  begin  as  before  with  the  simplest 
case. 

219.  Look  back  again  for  a  moment  at  the  view  and 
section  of  the  Raised  Beach  in  Figs  33  and  34.     The 
old  sand  and  gravel  beds  have  there  unquestionably 
been  raised  up  above  their  former  level,  but  they  have 
not  otherwise  been  disturbed.     They  still  lie  out  hori- 
zontally as  they  used  to  do.     But  would  this  be  the 
case  everywhere  along  that  terrace  ?     You  remember 
that  we  ascertained  that  the  terrace  cannot  be  traced 
all  round  the   country,   that   it   dies  out  in   certain 
directions,  and  consequently  that  the  elevation  which 
produced  it  was  not  universal  but  local.     Now  it  is 
clear  that  though  the  upheaved  tract  rose  so  uniformly 
that  the  raised  beach  may  retain  the  same  level  for 
many  miles,  still,  between  the  horizontal  strata  which 
were  upraised,  and  those  which,  lying  outside  of  the 


loS  SCIENCE  PRIMERS.  [THE  CRUST 

elevated  district,  remained  unchanged  in  level,  there 
must  be  an  intervening  space,  longer  or  shorter,  where 
the  strata  slope  down  from  the  raised  to  the  stationary 
ground. 

220.  To  make  this  clearer,  suppose  byway  of  illus- 
tration that  we  place  upon  a  table  a  number  of  sheets 
of  cloth  to  represent  the  different  strata  with  which  we 
are  dealing.     The  cloths,  like  the  strata,  lie  there  hori- 
zontally.     But  if  we   push   them   up  anywhere  they 
will  be  found  to  slope  away  from  the  elevated  to  the 
unmoved  part.     Put  a  flat  plate,  for  instance,  under- 
neath them,  so   as   to  raise    a  considerable  surface. 
Over  the  flat  surface  of  the  plate  the  cloths  are  flat, 
as  they  are  in  our  raised  beach,  but  from  that  upraised 
area  they  slope  down  to  the  undisturbed  parts  around. 
So  that  you  see  how  a   local  elevation,  even  though 
it   may  raise  up  strata   over  a  wide   district  without 
disturbing  their  flatness,    must   yet  give   rise    to  an 
inclination  of   the  strata   round  the  outskirts  of  the 
movement. 

221.  Wherever,  therefore,  strata  are  pushed  up  or 
let  down  more  at  one  place  than  at  another,  without 
being  actually  broken   across,   they  must  be   thrown 
into    an  inclined  position.      Now    this   unequal    and 
irregular  kind  of  movement  has   taken  place    many 
times  in  every  quarter  of  the   globe.     If  you  look  at 
the  stratified  rocks,  in   most   parts  of  this  and  other 
countries,  you   will    seldom    find   them    quite  flat — 
usually  they   are  inclined,  sometimes   gently,    some- 
times steeply,  so  that  they  have  not    only  been  up- 
heaved  out  of   the  sea  (Art.    206),  but   have   been 
moved  irregularly  and  unequally. 

222.  In    the   quarry   which    we    formerly    visited 


OF   THE   EARTH.] 


GEOLOGY. 


109 


(Art.  119)  the  strata  were  horizontal.  But  in  many 
quarries  you  would  find  them  turned  up  as  in  Fig. 
38,  where  the  right-hand  portion  has  gone  up  (or 


FIG.  ^8. — Inclined  Strata. 


the  left-hand  parts  have  gone  down)  more  than  the 
others.     In  some  places,  indeed,  you  will  meet  with 


FIG.  39.— Vertical  Strata. 

the  rocks  so  tilted  up  as  to  stand  fairly  on  end 
(Fig.  39),  like  a  row  of  books  on  a  shelf.  As  they 
are  made  of  sediment  which  gathered  on  a  flat  or 


SCIENCE  PRIMERS. 


[THE  CRUST 


gently  sloping  bottom,  you  see  at  once  that  they 
never  could  have  been  placed  on  end  originally,  but 
have  been  tilted  into  this  position  by  underground 
changes. 

223.  But  this  is  not  all.  If  when  the  cloths  were  lying 
flat  on  the  table  (Art.  220)  you  had  squeezed  them 
from  either  end,  they  would  have  been  thrown  into 
crumplings  (Fig.  40).  In  the  same  way  during  the 


FIG.  40.— Cloths  crumpled  by  pressure. 

movements  by  which  the  strata  have  been  raised  up  a 
great  deal  of  similar  crumpling  has  taken  place.  In 
Fig.  41,  for  instance,  the  hard  rocks  are  shown  to  have 
been  twisted  and  folded  over  as  if  they  had  been 
mere  layers  of  cloth.  How  enormous  must  have 
been  the  pressure  to  which  they  were  exposed  before 
they  were  squeezed  into  these  shapes  ! 

224.  One  difference  between  the  cloths  and  the  strata 
will  occur  to  you.  The  one  are  soft  and  pliable, 
the  others  hard  and  rigid.  But  we  may  make  even 
the  most  unyielding  rocks  to  bend  a  little,  and  if  this 


OF  THE  EARTH.  ] 


GEOLOGY. 


can  be  done  even  with  the  comparatively  feeble  force 
which  man  can  employ,  we  may  perceive  how,  under 
the  enormous  pressure  which  they  underwent  in  the 
depths  of  the  earth  before  they  were  upheaved,  the 
rocks  should  have  been  crumpled  up  like  mere  pliable 
layers  of  cloth. 

225.  Still  there  must  sometimes  have  been  a  point 
beyond  which  they  would  rather  break  than  bend  any 


FIG.  41.— View  of  contorted  strata. 

further.  Cracks  would  then  be  formed,  and  the  strata 
would  be  thrust  up  or  made  to  sink  down.  You  see  one 
of  these  cracks,  or  faults,  as  they  are  called,  at  /,  in 
Fig.  37,  where  the  coal-seams  and  the  strata  between 
them  have  been  broken  across — those  on  one  side  of  the 
fracture  being  now  found  at  a  lower  level  than  those 


SCIENCE  PRIMERS. 


[THE  CRUST 


on  the  other.  Dislocations  of  this  kind  are  of  such 
frequent  occurrence  that  the  whole  surface  of  the  earth 
may  be  looked  upon  as  a  network  of  cracks.  They 
greatly  interfere  with  the  working  of  coal-mines,  as 
shown  in  Fig.  37,  where  the  galleries  which  are  driven 
along  the  coal-seams  from  the  pit  towards  the  left-hand 
will  need  to  be  altered  where  the  coal  is  cut  off  by  the 
dislocation  / 

226.  It  has  often  happened  that  into  the  cracks  thus 
formed  masses  of  melted  or  igneous  rocks  from  the 


FlG.  42. — Section  of  Igneous  Rock  forced  UD  into  Cracks 
of  the  Larth's  Crust. 


C 

id  Fissures 


interior  have  been  pressed,  so  as  to  rise  up  and  inter- 
sect the  other  rocks.  In  the  section  in  Fig.  42,  for  ex- 
ample, two  such  dislocations  have  occurred  in  a  series 
of  stratified  rocks,  so  that  three  different  groups,  A,  B, 
and  C,  have  been  displaced.  Into  one  of  these  cracks 
a  mass  of  igneous  rock  (I)  has  forced  its  way  for  some 
distance.  But  in  the  other,  that  to  the  right  hand, 
a  much  larger  body  of  melted  rock  has  risen  so  as 
completely  to  separate  the  stratified  rocks  B  and  C, 
and  not  only  so,  but  to  break  through  the  group  B, 


OF   THE   EARTH.]  GEOLOGY.  113 

ascending  even  to  what  is  now  the  surface  of  the 
earth. 

IV.  The  Origin  of  Mountains. 

227.  It  is  common  to  speak  of  the  "eternal  hills" 
as  if  they  had  existed  from  the  very  beginning  of  the 
world's  history.     And  certainly  few  objects  upon  the 
surface  of  the  globe  convey  to  the  mind  such  an  impres- 
sion of  vast  antiquity.     As  far  back  as  history  or  tradi- 
tion can  go  the  mountains   have   remained  without 
sensible  change ;  and  thus,  because  they  have  always 
appeared  to  man  to  be  what  they  still  are,  he  is  apt  to 
think  of  them  as   parts  of  the  original  architecture 
of  the  planet. 

228.  And  yet  from  what  has  been  learned  in  some 
of  the  foregoing  Lessons,  you  will  be  prepared  to  find 
that  old  as  the  mountains  undoubtedly  are,  they  do  not 
belong  to  the  beginning  of  things.     It  is  still  possible 
to  trace  out  their  origin,  and  to  get  back  to  the  records 
of  earlier  times  before  they  existed  at  all.     If  I  ask 
how  this  knowledge  can  he  gained  you  will  doubtless 
answer  that  it  can  only  be  by  examining  the  rocks  of 
which  the  mountains  consist.     You  have  already  learnt 
how  rocks  tell  their  story.     It  is  only  a  further  stage  of 
the  same  kind  of  reasoning  to  inquire  what  the  rocks 
tell  regarding  the  birth  of  the  mountains. 

229.  First  of  all,  then,  when  any  chain  of  mountains 
is  examined  it  is  found  to  be  made  of  rocks  belong- 
ing to  one  or  more  of  the  three  great  classes — with 
which   you   are    already  acquainted.     In  particular, 
the  great  mass  of  most  mountain  chains  consists  of 
various  kinds  of  stratified  rocks — such  as  sandstones, 


ii4  SCIENCE  PRIMERS.  [THE  CRUST 

conglomerates,  limestones,  and  others.  Now  you  have 
found  that  these  rocks  have  been  laid  down  under 
water,  most  of  them  under  the  sea.  They  often 
contain  the  remains  of  shells,  corals,  sea-urchins,  or 
other  marine  creatures,  and  these  remains  may  be 
taken  out  of  the  rocks  even  at  the  summits  of  the 
mountains  (Art.  160).  No  clearer  proof  than  this 
could  be  required  to  show  that  mountains  are  not  so 
old  as  "the  beginning  of  things,"  for  these  fossils 
prove  that  where  the  mountains  now  stand  wide  seas 
once  rolled. 

230.  Again,  mountains  which  consist  of  rocks  formed 
originally  under  the  sea  must  owe  their  existence  to 
some  force  which  could  raise  up  the  bed  of  the  sea 
into  high  land.  That  force  has  been  already  (Art. 
187)  alluded  to.  As  a  consequence  of  the  slow  cool- 
ing of  our  planet,  its  outer  crust,  under  the  enormous 
strain  of  contraction,  has  been  forced  up  into  ridges 
in  different  places,  with  wide,  sunk  spaces  between. 
The  ridges  form  mountain  chains,  while  the  sunk 
spaces  are  filled  with  the  waters  of  the  ocean.  If  you 
look  at  a  map  of  the  world  you  may  trace  out  the 
principal  lines  of  elevation,  as  they  are  called, 
over  the  globe.  Perhaps  the  most  remarkable  of  all 
the  folds  or  puckerings  into  which  the  surface  of  the 
earth  has  been  ridged  up  is  the  long  line  of  moun- 
tains which  runs  down  the  whole  of  the  continent  of 
America.  You  observe  that  the  various  ridges  of  the 
Rocky  Mountains,  of  Central  America,  and  of  the 
Cordilleras  and  Andes,  are  prolonged  in  one  vast  line 
of  elevation.  Other  minor  foldings  are  seen  on  the 
same  continent,  as,  for  instance,  the  chain  of  the 
Alleghanies,  in  the  eastern  part  of  the  United  States. 


OF   THE   EARTH.]  GEOLOGY.  115 

]n  Europe  we  have  a  line  of  elevation  stretching 
across  the  continent,  and  throwing  off  spurs  in  its 
course.  It  is  seen  in  the  Pyrenees,  then  in  the  Alps, 
whence,  after  sending  southwards  the  ridges  of  the 
Apennines,  it  is  carried  eastward  by  the  chain  of  the 
Carpathians,  and  then  by  the  Caucasus  to  the  Caspian 
Sea.  The  same  line,  however,  reappears  on  the  other 
side  of  that  inland  sea,  and  crosses  the  vast  continent 
of  Asia  in  two  divergent  lines ;  one  of  which  turns 
south-eastward,  to  form  the  grand  Himalayas,  while 
the  other  trends  eastward  across  the  great  Asiatic 
table-land  to  the  shores  of  the  Pacific  Ocean.  When 
you  reflect  upon  these  enormous  mountain-chains  as 
the  results  of  the  cooling  and  contraction  of  the 
mass  of  the  globe,  you  begin  to  feel  how  enormous 


FIG.  43. — Section  of  a  series  of  Sedimentary  Rocks  originally  deposited 
horizontally  on  the  sea-bottom. 

must  be  the  force  which  could  crumple  up  solid  rock 
into  ridges  many  thousands  of  miles  long  and  thou- 
sands of  feet  high. 

231.  But  as  the  globe  has  been  cooling  and  con- 
tracting since  the  very  first,  we  may  reasonably  expect 
to  find  that  mountains  have  been  uplifted  at  various 
limes,  and,  therefore,  that  they  differ  from  each  other 
in  age.     A  little  attention  to  the  rocks  is  enough  to 
show  not  only  that  mountains  are  not  all  of  the  same 
age,  but  that  even  the  same  mountain  has  not  been 
formed  entirely  at  one  time,  but  that  one  part  has  been 
raised  up  long  before  another. 

232.  Suppose,  for  example,  that  a  series  of  ordinary 


u6 


SCIENCE  PRIMERS. 


[THE  CRUST 


sedimentary  rocks,  such  as  the  sandstones,  conglome- 
rates, and  shales,  described  in  earlier  Lessons,  has  been 
laid  down  upon  the  sea-bottom.  These  rocks  would 
be  formed  one  above  another  in  flat  beds  (Fig.  43), 
until  they  had  accumulated  into  a  mass  perhaps  many 
thousands  of  feet  in  thickness.  They  might  remain 
undisturbed  for  a  long  time.  Let  us  further  suppose, 


FIG.  44.  —Section  of  a  mountain  formed  of  crumpled  rocks,  A,  which  have 
been  contorted  before  the  deposition  of  the  flat  rocks,  B. 

however,  that  they  happen  to  lie  in  one  of  those 
weaker  parts  of  the  crust  which,  when  the  accumulated 
effects  of  the  long-continued  contraction  of  the  earth's 
mass  begin  to  make  themselves  felt,  are  pushed  out- 
ward by  the  subsiding  spaces  on  either  side.  Squeezed 
together  by  the  pressure  of  these  sinking  areas  the 


FIG.  45. — Section  of  a  mountain  in  which  the  rocks  A  were  upheaved  before 
the  series  B,  and  the  latter  before  series  c. 

formerly  horizontal  rocks  will  be  crumpled  up  into 
folds  (like  our  cloths  in  Fig.  40,  when  similarly 
squeezed),  and  be  made  to  rise  above  the  level  of  the 
surrounding  tracts  (Fig.  44).  A  ridge  or  mountain- 
chain  would  thus  arise  upon  the  surface  of  the  earth. 


OF   THE   EARTH.]  GEOLOGY. 


233.  Such  a  ridge  or  chain  formed  out  of  sedimen- 
tary rocks  (A),  once  horizontal  but   now   contorted, 
could  not  rise  up  into  the  atmosphere  without  becom- 
ing a  prey  to  those  various  forces,  which,  as  you  have 
learnt  (Physical  Geography  Primer,  Arts.   126 — 142), 
are  ceaselessly  at  work  in  wearing  down  the  surface  of 
the  globe.     The  air,  rain,  springs,  rivers,  frosts,  or  the 
waves  of  the  sea,  would   attack   the   newly  formed 
mountain,  and  begin  to  waste  its  surface  as  soon  as 
it  raised  its  head  above  the  level  of  the  ocean.     Deep 
furrows  would  in  time  be  carved  out  of  its  sides,  and 
all  its  decayed  fragments  would  be  washed  down  to 
the   lower  grounds.      There   these  fragments  would 
form  new  deposits,  which  would  be  laid  down  upon 
the  edges  of  the  older  rocks,  as  in  Fig.  44  the  newer 
series  B  is  seen  to  lie  upon  the  older  A. 

234.  Now  such  a  section  as  this  (Fig.  44)  would 
enable  you  to  fix,  relatively  at  least,  the  date  of  the 
mountain.     You  could  assert  positively  that,  ist,  there 
was  a  time  when  the  mountain  did  not  exist,  but  when 
its  place  was  occupied  by  a  sea  in  which  the  sedi- 
mentary rocks  A  were  deposited ;  2nd,  that  the  moun- 
tain was  formed  by  the  crumpling  up  of  these  rocks, 
and  that  this  took  place  before  any  of  the  rocks  of 
series  B  began  to  be  formed ;  and  3rd,  that  after  the 
formation  of  the  strata  marked  B  the  whole  mass  was 
further  uplifted  so  as  to  raise  these  strata  out  of  water 
into  dry  land. 

235.  But  suppose  that  in  some  other  part  of  the 
chain  we  discover  such  an  arrangement  of  rocks  as 
that  shown  in  Fig.  45.     Here,  as  before,  we  see  that 
the  series  A  was  upturned  before  the  series  B  could 
be  laid  down  on  it.     But  in  the  present  case  series  B 


ii8  SCIENCE  PRIMERS.  [THE  CRUST 

has  also  been  tilted  up  out  of  its  original  horizontal 
position.  Such  a  mountain  would  indicate  three  suc- 
cessive periods  of  upheaval,  the  first  older  than  the 
time  of  B,  the  second  older  than  the  time  of  c,  while 
the  third  came  after  the  formation  of  c,  for  it  raised 
that  series  of  strata  into  land. 

236.  It   is  in  this   kind  of  way  that  the   relative 
ages  of  mountain-chains  are  determined.     Wherever 
you  meet  with  sedimentary  rocks  turned  up  on  end  or 
crumpled,  you  know  that  they  have  been  disturbed, 
and  whenever  these  disturbed  rocks  have  their  broken 
edges  covered  by  others,  you  see  that  the  uplifting 
must  be  older  than  the  second  set  of  rocks. 

237.  If  now  you  could  find  out  any  means  of  recog- 
nizing the  same  series  of  rocks  in  different  countries  : 
if,  for  example,  you  could  be  sure  that  the  groups  A 
and  B  of  Figs.  44  and  45  occurred  both  in  England  and 
Germany,  you  would  be  able  to  compare  the  relative 
ages  of  the  mountains  of  the  two  countries.    If  in  the 
one  country  a  mountain  shows  the   structure  repre- 
sented in  Fig.  45,  and  if  in  the  other  country  a  moun- 
tain formed  of  the  same  series  of  rocks  is  built  up  in 
the  way  shown  in  Fig.  44,  you  would  infer  that  the 
former    mountain  was    newer  than,    or,    rather,    had 
received  a  push  upward  after,  the  latter. 

238.  In  the  next  Lesson  it  will  be  shown  how  geo- 
logists identify  the  same  series  of  rocks  in  different 
countries — viz.  by  Fossils.    With  this  kind  of  evidence 
it  becomes  possible  to  decide  which  are  the  oldest  and 
which  the  newest  mountain-chains.     In  this  way  we 
learn  that  the  giant  Alps,  towering  so  far  above  the 
plains  of  Europe,  are  less  ancient  than  many  a  green 
hill  in  Wales  and  Scotland. 


OF   THE   EARTH.] 


GEOLOGY. 


119 


239.  But  another  singular  and  important  fact  about 
mountains  is  made  plain  by  such  sections  as  those  in 
Figs.  44  and  45.  The  series  of  rocks  marked  A  is  the 
oldest  part  of  the  mountain  in  each  case.  You  might 
naturally  suppose  that  the  oldest  parts  ought  to  be 
those  which  are  buried  deep  under  other  portions. 
"And  yet  on  examination  it  is  found  that  the  most 
ancient  parts  are  not  always  those  which  lie  at  the 


lowest  level,  but  that,  as  in  the  two  cases  we  have 
supposed,  they  may  have  been  pushed  up  so  as  now 
actually  to  form  the  loftiest  peaks  and  ridges.  But  if 
you  get  away  to  the  flanks  of  the  mountain  you  find 
that  the  oldest  rocks  do  really  pass  under  the  newer 
ones,  as  series  A  in  the  drawings  passes  under  B. 
240.  The  crumbling  down  of  the  surface  of  the 


120  SCIENCE  PRIMERS.  [THE  CRUST 

earth  is  so  constant  and  so  wide-spread,  that  in  pro- 
cess of  time  every  mountain-chain  undergoes  great 
and  manifold  changes.  Its  summits  and  sides  are 
wasted  and  lowered.  Its  crests  get  splintered  into 
peak  and  pinnacle,  as  the  rains  and  frosts  of  ages 
do  their  work  upon  them.  Crag  and  cliff  are  carved 
out  of  its  sides  ;  ravine  and  gorge,  wider  glen  and  still 
wider  valley,  are  excavated  in  its  rocks  by  the  never- 
ending  flow  of  rill  and  river.  Hence,  though  the 
original  line  of  elevation  remains,  the  upheaved  tract 
is  cut  into  innumerable  ridges  and  valleys  as  the  pro- 
cess of  waste  goes  on.  (See  Physical  Geography 
Primer,  Art.  126.) 

241.  So  enormous  have  been  the  effects  of  that 
process  over  the  surface  of  the  earth,  that  great  table- 
lands or  broad  masses  of  high  land  have  been  cut  down 
into  mere  ridges  and  detached  hills.  In  Fig.  46  you 
may  as  it  were  watch  how  the  excavation  proceeds. 
That  is  a  drawing  of  part  of  a  table-land  in  Spain. 
You  observe  how  the  streams  as  they  descend  and 
become  larger  in  size,  dig  wider  and  deeper  trenches  out 
of  the  rocks,  how  their  ravines  widen  out  into  valleys, 
how  the  high  ground  between  them  is  being  cut  into 
irregular  ridges,  and  how  these  ridges,  still  further  cut 
into  separate  hills  or  mounds,  lose  height  as  the  rains 
and  frosts  attack  their  summits  and  sides.  In  every 
quarter  of  the  globe  illustrations  of  these  changes 
may  be  found.  In  Britain,  for  example,  our  mountains 
are  only  fragments,  like  those  in  the  drawing,  which 
have  been  left  after  the  excavation  of  the  valleys  that 
surround  them.  The  great  Ghauts  of  India,  and  the 
Table  Mountain  of  the  Cape,  are  likewise  conspicuous 
instances  of  the  same  kind  of  origin. 


OF   THE   EARTH.]  GEOLOGY. 


242.  The    same   forces    which    have    carved    out 
valleys  and  left  mountain-ridges  standing  out  between 
them  are  still  busy  at  their  work.     Every  year  adds  to 
the  waste.     And  thus,  although  when  we  gaze  at  a 
mountain-chain  we  know  that  first  of  all  it  was  heaved 
up  by  movements  from  below,  we  nevertheless  learn  to 
recognize  that  all  the  familiar  forms    which  it  now 
assumes  have  since  that  early  time  of  upheaval  been 
carved  upon  it  by  the  very  same  forces — rains,  frosts, 
springs,  glaciers,  and  the  rest — which  are  busy  sculp- 
turing its  surface  still. 

V.— How  the  Rocks  of  the  Crust  tell  the 
History  of  the  Earth. 

243.  When  a  historian  betakes  himself  to  write  the 
history  of  a  country,   his  first  care  is  to  make  him- 
self acquainted   with   all    the    scattered    documents 
likely  to    throw   light  upon   the   transactions   which 
he  will  have  to  describe.     He  ransacks  the  papers 
in  the  public  archives  and  libraries,  gleans  what  he 
can  from  printed  books,  and  even  it  may  be  travels 
into   foreign   countries    in   search    of   contemporary 
writings    which    may    explain    what   is    dark  or    un- 
certain at   home.     Only    after    prolonged   labour  of 
this  kind,  is  he  able  to  gather  up  the  sum  of  all  he 
has  learnt,  and  to  weave  it  into  a  continuous  narrative. 
In  the  course  of  his  researches  he  will  no  doubt  find 
some  periods  much  better  illustrated  by  contempo- 
rary documents  than  others,  while  of  some  he  may 
possibly  be   hardly  able   to   collect   any  satisfactory 
information,   for   in   the   course   of  time  the  papers 
which  would  have  supplied  him  with  the   facts  are 


122  SCIENCE  PRIMERS.  [THE  CRU^T 

lost  or  destroyed.  Hence  his  history  is  not  all 
equally  full  and  reliable.  There  may  even  be  gaps 
in  it  which  no  amount  of  industry  in  his  search  for 
information  has  enabled  him  to  fill  up. 

244.  Now  what  is  thus  true  of  the  historian  of  any 
country,  is  true  also  of  the   geologist.     As   already 
pointed  out  (Art.  38),  and  as  must  now  be  very  clear 
to  you,  the  earth  has  a  history  as  well  as  the  people 
who  dwell  upon   its    surface.     A  geologist   may   be 
called  a  historian  of  the  earth.     His  great  aim  is  to 
collect  all  the  evidence  which  remains  of  the  changes 
that  have  happened  upon  the  earth's  surface,  and  to 
arrange  them  in  the  order  in  which  they  have  occurred, 
so  as  to  show  the  grand  march  of  events  down  to  the 
present  time. 

245.  What  papers  and  inscriptions,  coins  and  books 
are  to  the  historian,  the  rocks  of  the  earth's  crust  are 
to  the  geologist.    They  contain  all  the  real  evidence  at 
his  disposal.     What  he  can  gather  from  them  at  one 
place  must  be  compared  with  what  he  collects  from 
them  at  another.     He  must  journey  far  and  wide  in 
search  of  facts  which  are  not  to  be  found  at  his  own 
door.     Gaps  will  certainly  occur,  which  even  the  skill 
and  industry   of  many   years  may  never  completely 
bridge  over ;  for  the  rocks,  as  we  have  already  seen, 
are  subject  to  revolutions  quite  as  destructive  in  their 
way,  as  those  which  have  swept  away  the  archives  of 
cities  and  nations.     The  geologist,  therefore,  can  only 
at  the  best  produce  an  imperfect  chronicle.     But  it 
is  one  which  has  a   profound  interest  for  all  of  us,  for 
it  is  the  story  of   our  own  globe — of  its  continents 
and  oceans,  its  mountains  and  valleys,  its  rivers  and 
lakes,  of  the  tribes  of  plants  and  animals  which  people 


OF   THE   EARTH.]  GEOLOGY.  123 

its  surface,  and  of  the  advent  and  progress  of  man 
himself. 

246.  Regarding  the  earliest  stages  of  the  earth's  his- 
tory no  direct  evidence  is  now  to  be  obtained  from  the 
rocks.     When  the  earth  was  detached  from  its  parent 
sun,  it  must  have  been  a  fiercely  hot  mass  as  the  sun  is 
still.     Not  until  long  after  that  period  could  any  such 
rocks  as  we  now  see  have  been   formed.     So   that 
although  the  rocks  carry  us  a  vast  way  back  into  the 
past,  they  cannot  bring  us  to  the  beginning  of   the 
earth's  history  as  a  separate  planet.     That  early  time 
can   only  be   inferred   from  other  and  chiefly  astro- 
nomical evidence. 

247.  In  the  foregoing  pages  you  have  learnt  how 
the  rocks  m'ay  each  be  made  to  give  up  its  little  bit 
of  earth-history.     You  succeeded  in  discovering,  for 
example,  from  the  rocks  of  a  single  quarry  the  site  of 
an  old  sea-floor  with  some  of  the  remains  of  the  sea- 
creatures  which  lived  upon  it  (Arts.  118 — 131).    Again, 
you  found  how  a  peat-moss  might  enable  you  to  trace 
out  the  limits  of  a  long-vanished  lake  on  which  our 
rude  forefathers  launched  their  oak  canoes  (Arts.  144 
— 152) ;  and  how  the  rocks  of  a  coal-pit  could  furnish 
forth  a  record  of  forest  after  forest  which  had  each 
flourished  green    and    fair  at  the  surface,  but  which 
had  sunk  down  one  after  the  other,  and  were  now 
buried  deep  within  the -earth.     (Arts.  137  and  212.) 

248.  In  these  and  all  such  illustrations,  while  each 
series  of  rocks  tells  its  own  story,  that  story  is  only  a 
part  of  the  general  history  of  the  earth.     The  more 
carefully  we  can  gather  each  separate  narrative,  the 
fuller  will  be  that  general  chronicle  of  the  earth's  his- 
tory which  it  is  the  object  of  geology  to  compile. 


124  SCIENCE  PRIMERS.  [THE  CRUST 

249.  According  to  the  law  of  superposition  (Art. 
122)  the  undermost  stratified  rocks  are  the  oldest.  We 
can  reach  but  a  little  way  down  into  the  earth.     The 
deepest  mines  descend  but  a  very  few  thousand  feet 
into  the   rocks.     If,   therefore,    these  rocks  still  lay 
flat  as  they  were  deposited,  we  should  be   able  to 
make  ourselves  acquainted  only  with  those  near  the 
surface.     But  in  consequence  of  the  way  in  which  the 
rocks  have  been  bent  and  broken,  and  pushed  up  and 
down  (Arts.    188,    218 — 226),   we  not  only  see   the 
topmost  parts  of  the  series,   but   even  some  of  the 
oldest  masses.     Instead  of  lying  flat,  the  rocks  are 
very  commonly  found  to  slope  into  the  earth  more  or 
less  steeply,   and  we   can  walk  over  their-  upturned 
edges,  like   the  backs    of  so   many   rows   of  books. 
(See    Figs.  38   and    39.)      So  far  therefore  from   the 
bottom  rocks  being  still  buried  under  the  thousands 
of  feet   of  solid  rock  beneath  which   they  once  lay, 
they  are  often  found  rising  into  the  summits  of  lofty 
mountain  ranges  (Art.    239).     And  thus  the  geologist 
is  saved  the  trouble  of  sinking  deep  bores  and  pits  to 
find  out   the  order  of  the  rocks  under  his  feet.     By 
making  careful  sections  from  what  can  be  observed  at 
the  surface  (as  in  Figs.  44  and  45),  he  determines  that 
order  with  certainty,   and  when  he  has  done  so  he 
knows  which  are  the  oldest  parts  of  his  chronicle,  and 
which  are  the  newest. 

250.  The  crust  of  the  earth,  so  far  at  least  as  we  can 
examine  it,  is  chiefly  made  up  of  Sedimentary  and 
Organic  Rocks.     In  these  rocks,  therefore,  must  the 
chief  sources  of  evidence  for  the  history  of  the  earth 
be  sought.     If  we  could  pile  them  up,  one   above 
another,  in  the  order  of  their  formation,  they  would 


OF   THE   EARTH.]  GEOLOGY.  125 


form  a  mass  probably  more  than  a  dozen  miles  thick. 
This,  then,  is  the  library  out  of  which  geological 
history  must  be  compiled. 

251.  Besides  the  order  of  superposition,  however,  the 
geologist  has  another  clue  to  the  relative  age  of  rocks. 
By  comparing  the  different  series  of  rocks  with  each 
other  he  has  discovered  that  the  fossils,  or  remains  of 
plants  or  animals  (Art.  117),  of  one  series  differ  from 
those  of  another.  For  example,  to  turn  again  to  Fig. 
45,  it  is  ascertained  that  if  fossils  occur  in  the  set  of 
rocks  marked  A,  they  will  be  found  to  differ  from 
those  in  the  series  B,  and  these  again  from  those  in  c. 
If,  starting  from  the  plants  and  animals  of  to-day,  we 
go  back  into  older  and  yet  older  rocks,  we  learn  that 
the  fossil  plants  and  animals  become,  on  the  whole, 
more  and  more  unlike  those  which  are  still  living. 
Each  great  division  of  rocks  is  found  to  have  its  own 
characteristic  fossils.  So  that,  over  and  above  the  test 
by  Order  of  Superposition,  we  can  discriminate 
between  these  divisions  by  means  of  Fossils. 

252.  By  these  methods  of  classification  the  vast  com- 
plex mass  of  Stratified  Rocks  may  be  divided  into  a 
few  great  divisions,  these  into  subdivisions,  these  again 
into  minor  compartments,  and  these  into  still' smaller 
zones  or  bands,  so  that  when  a  bed  of  rock  is  found 
it  can  be  referred  to  its  own  particular  part  of  the 
whole  vast  series.     This  method  of  arrangement  is 
necessary  for  the  sake  of  clearness,  very  much  in  the 
same  way  that  a  work  on  history  requires  to  be  divided 
into  volumes,  these  into  separate  books,  these  again 
into  chapters,  and  these  into  pages  and  lines. 

253.  Making  use,  therefore,  of  every  kind  of  evi- 
dence which  the  rocks  afford,  the  geologist  endeavours 


126  SCIENCE  PRIMERS.  [CONCLUSION. 

to  weave  together  his  narration  of  the  history  of  the 
earth.  He  shows  h'ow  land  and  sea  have  often 
changed  places,  how  time  after  time  volcanos  have 
broken  out  in  all  quarters  of  the  globe,  how  continents 
have,  one  by  one,  arisen,  how  mountain-chains  have 
been  successively  formed,  how  valleys,  and  ravines, 
and  lakes,  have  been  excavated,  how  climates  have 
slowly  changed  from  tropic  heat  to  arctic  cold.  Amid 
all  these  revolutions  of  the  solid  earth  itself,  he  finds 
that  there  have  been  at  the  same  time  vast  changes  in 
the  plants  and  animals  which  have  peopled  its  surface. 
He  can  trace  how  Life,  beginning  in  the  remotest 
past  with  the  simplest  organisms,  has  advanced  through 
long  ages,  in  more  and  more  highly  organized  forms 
(Art.  132),  up  to  the  present  time.  He  can  mark  how 
group  after  group  of  shells,  or  fishes,  or  reptiles,  has 
come  into  existence,  and,  after  living  for  protracted 
periods,  has  slowly  died  out  to  make  way  for  newer 
tribes,  until  towards  the  close  of  the  history  Man  has 
appeared  upon  the  scene. 

254.  Geological  history  brings  before  us,  in  this  way, 
many  facts  well  calculated  to  impress  our  minds  with 
the  great  antiquity  of  our  planet,  and  with  the  mar- 
vellous chain  of  changes  by  which  the  present  order 
of  things  has  been  brought  about.  We  learn  from  it 
that  mountains  and  valleys  have  not  come  suddenly 
into  existence,  such  as  we  now  see  them,  but  have 
been  formed  gradually,  by  a  long  series  of  processes 
similar  to  those  which  are  even  now  slowly  doing  the 
same  work.  We  discover  that  every  part  of  the  land 
under  our  feet  can  yield  us  up  its  story,  if  we  only  know 
how  to  question  it.  And,  strangest  of  all,  we  find 
that  the  races  of  plants  and  animals  which  now  tenant 


CONCLUSION.]  GEOLOGY.  127 

land  and  sea,  are  not  the  first  or  original  races,  but 
that  they  were  preceded  by  others,  these  again  by 
others  still  more  remote.  We  see  that  there  has  been 
upon  the  earth  a  history  of  living  things,  as  well  as  of 
dead  matter.  At  the  beginning  of  that  wonderful 
history  we  detect  traces  merely  of  lowly  forms,  like 
the  foraminifera  of  the  Atlantic  ooze.  At  the  end  we 
are  brought  face  to  face  with  Man — thinking,  working, 
restless  Man,  battling  steadily  with  the  powers  of 
Nature,  and  overcoming  them  one  by  one,  by  learning 
how  to  obey  the  laws  which  direct  them. 

CONCLUSION. 

255.  It  is  not  the  design  of  this  little  book  to  enter 
further  into  the  history  of  the  Earth.     It  has  led  you 
to   the   threshold   whence  you  can  see  the  kind  of 
interest  in  store  for  you  if  you  advance  beyond.     You 
have  now  learnt  something  of  the  general  principles 
upon  which  the  history  is  based.     Looked  at  in  the 
light  of  geological  teaching,  the  very  stones  of  the 
street  and  the  pebbles  of  the  shore  have  each  a  mean- 
ing for  you  now.     You  will  no  longer  be  content  to 
gather  minerals  and  rocks  merely  because  they  are 
pretty  objects  to  look  at.     Apart  from  their  beauty 
you  will  seek  to  discover  what  they  are,  and  how  they 
came  to  be  where  they  are  found. 

256.  A  landscape  will  lose  none  of  its  beauty  in  your 
eyes  though  you  seek  to  discover  how  the  rocks  of  its 
hills  were  formed,  how  ridge  and  valley  came  into 
existence,  why  a  crag  should  rise  in  one  part  and  a 
wide  plain  stretch  away  for  miles  in  another.     When 
you  stand  by  the  brink  of  a  foaming  river  there  will 

12 


128  SCIENCE  PRIMERS.          [CONCLUSION. 

be  no  lessening  of  your  pleasure  in  its  rush  and  roar, 
if  you  think  of  the  river  as  one  of  nature's  most 
powerful  engines,  busy  day  and  night  in  digging  out 
its  channel  in  the  rocks,  and  carrying  the  waste  of  the 
mountains  down  to  the  plains  and  to  the  depths  of  the 
ocean.  The  shores  of  the  sea  will  wear  a  new  charm 
when  you  trace  along  their  cliffs  and  caves  the  progress 
of  decay,  and  on  their  beaches  of  sand  and  shingle  the 
counterpart  of  those  sedimentary  deposits  out  of  which 
whole  mountains  are  built  up. 

257.  Every  quarry  and  ravine  where  the  naked  rock 
comes  to  view,   offers  an  attraction,  if  you  seek  to 
find  there  the  remains  of  some  of  those  lost  forms  of 
plants  which  covered  the  land,  or  of  those  long  extinct 
tribes  of  animals  which  once  tenanted  the  sea.     These 
fossils  will  become,  in  your  hands,  not  mere  things  to 
wonder  at.      ^ou  will  try  to  learn,  from  friend  or  book, 
what  they  resemble  most  in  the  present  living  world. 
And  you  will  not  rest  contented  until  you  have  seen 
all  that  you  can  discover  of  the  light  which  they  throw 
upon  the  former  condition  of  the  district  in  which  you 
find  them. 

258.  Geology  will  thus  be  no  longer  a  task  to  be 
conned  from  books,  but  a  delightful  companion  in  every 
walk  and  ramble.     You  may  never  become  geologists, 
but  you  will  never  regret  the  time  you  have  spent  in 
trying  to  master  the  principles  on  which  geological 
science  is  based,  and  to  trace  out  under  their  guidance 
the  marvellous  History  of  the  Earth. 


QUESTIONS, 

INTRODUCTION. 

1.  Mention  some  of  the  most  familiar  kinds  of  stone  and  the 
uses  to  which  they  are  applied. 

2.  What  are  brick  and  moitar  made  of? 

3.  How  is  iron  obtained  ? 

4.  Whence  are  limestone,  slate,  marble,  and  coal  obtained? 

5.  Under  what  kind  of  covering  do  the  rocks  of  a  country  for 
the  most  part  lie  ? 

6.  What  is  the  general  character  of  the  surface  of  Britain 
along  a  line  drawn  from  Liverpool  to  Harwich  ? 

7.  What  is  the  general  character  of  the  surface  of  Britain 
along  a  line  drawn  from  Skye  to  Montrose? 

8.  Explain  why  this  difference  of  character  should  exist  in  two 
parts  of  the  same  country. 

DIFFERENT  KINDS  OF  STONES,  p.  6. 

1.  What  is  meant  by  a  principle  of  classification  ? 

2.  Show  how  mere  colour  or  hardness  and  softness  would  not 
be  sufficient  as  a  principle  of  classification  of  stones. 

3.  Describe  the  characters  of  a  piece  of  sandstone. 

4.  From  these  characters  how  would  you  define  a  sandstone  ? 

5.  Describe  in  the  same  way  the  characters  of  a  piece  of 
granite. 

6.  Give  a  definition  of  granite  from  these  characters. 

7.  Describe  the  characters  of  a  piece  of  chalk,   and  explain 
how  the  examination  of  the  specimen  should  be  carried  on. 

8.  Give  a  short  definition  of  chalk  from  these  characters. 

WHAT  STONES  HAVE  TO  TELL  US,  p.   14. 

I.  Are  the  different  kinds  of  stone  scattered  at  random  over 
the  surface  of  a  country?  Illustrate  this  answer  by  reference  to 
the  stones  which  underlie  the  soil  of  Britain. 


130  SCIENCE  PRIMERS. 

2.  What  kind  of  history  is  made   known  to  us  by  stones? 
Give  illustrations  from  the  stones  of  Britain. 

3.  What  is  the  subject  of  the  science  of  Geology? 

SEDIMENTARY  ROCKS. 
I.  What  Sediment  is,  p.   19. 

1.  Into  what   groups   may  the  different   kinds   of  stone  be 
divided  ? 

2.  Define  the  sense  in  which  the  word  "rock"  is  used  in 
Geology. 

3.  What  is  sediment? 

4.  What  are  sedimentary  rocks? 

5.  Describe  a  piece  of  conglomerate,  and  show  out  of  what 
materials  it  has  been  formed. 

6.  Of  what  materials  do  sandstone  and  shale  consist? 

7.  What  two   questions   about   their   origin   do   sedimentary 
rocks  suggest  to  us  ? 

II.  How  Gravel,  Sand,  and  Mud  are  made,  p.  23. 

1.  What  question  is  it  well  to  put  to  ourselves  when  we  try  to 
find  out  the  history  of  any  kind  of  rock?     [Art.  57.] 

2.  What  is  the  difference  between  gravel  and  cand,  and  how 
may  this  be  shown  ? 

3.  Describe  the  origin  of  the  rubbish  which  covers  the  slopes 
of  mountains  and  high  hills. 

4.  This  rubbish  has  usually  sharp-edged  fragments  which  get 
more   and    more   rounded  as    they   descend    the   neighbouring 
streams.      Explain  this  change. 

5.  Why  does  fine  mud  travel   further   down  a  stream  than 
coarse  gravel  ? 

6.  In  what  form  is  the  debris  of  the  mountains  strewn  over 
the  plains? 

7.  On  a  rocky  sea-coast  what  is  the  difference  between  the 
surface  of  the  rocks  of  the  cliffs  and  those  of  the  beach  ?   Explain 
the  cause  of  this  difference. 

8.  What  becomes  of  the  fragments  which  fall  from  the  face  of 
a  sea-cliff? 

III.  How  Gravel,  Sand,  and  Mud  become  Sedimentary 
Rocks,  p.  32. 

i.  What  is  the  relation  between  the  rate  of  motion  of  a  stream 
and  the  deposit  of  sediment  at  the  bottom  oi  the  water? 


GEOLOGY.  131 


2.  What  would  you  infer  from  beds  of  gravel,  of  sand,  and  of 
mud  as  to  the  rate  of  movement  of  the  water  in  which  ^these 
deposits  are  laid  down  ? 

3.  How   would  you  apply  your  knowledge  of  the  origin    of 
gravel  and  mud  to  the  history  of  rocks  like  conglomerate  and 
shale? 

4.  Where  and  in  what  way  are  sedimentary  materials  arranged 
by  rain  upon  a  roadway  ? 

5.  How  is  the  sediment  disposed  of  by  the  Rhone  at  the  Lake 
cf  Geneva? 

6.  What  becomes  of  the  sand  and  mud  brought  down  by  a 
river  to  the  sea  ? 

7.  Define  the  terms  Stratification  and  Stratified  Rocks. 

8.  Why  have  sedimentary  rocks  usually  become  harder  than 
they  originally  were  ?     Explain  the  terms  Pressure  and  Infiltra- 
tion with  reference  to  the  history  of  those  rocks. 

9.  Define  a  Sedimentary  Rock. 

IV.  How  the  Remains  of  Plants  and  Animals  come  to 
be  found  in  Sedimentary  Rocks,  p.  ^4. 

1.  How   are   the  remains   of   land -plants  buriedin   modern 
sedimentary  deposits? 

2.  Explain  how  plants  have  been  often  imbedded  in  sandstone 
and  shale. 

3.  Explain  the  origin  of  the  remains  of  marine  animals  in 
many  shales  and  limestones. 

4.  Why  may  we  expect  any  marine  sedimentary  deposits  to  be 
full  of  traces  of  once-living  creatures  ? 

5.  What  is  a  fossil  ? 

V.  A  Quarry  and  its   Lessons,  p.  50. 

1.  What  is  usually  the  most  obvious  feature  in  a  quarry  among 
stratified  rocks? 

2.  W'hich  are  the  oldest  of  the  different  beds  in  the  quarry,  and 
why? 

3.  Define  the  term  Order  of  Superposition. 

4.  What  are  ripple-marks  in  rocks,  and  what  light  do  they 
cast  on  the  history  of  the  rocks  among  which  they  occur  ? 

5.  What  are  rain-prints  in  rocks,  and  what  evidence  do  they 
furnish  as  to  the  conditions  under  which  the  rocks  were  formed  ? 

6.  How   could  you  determine  from  the   evidence  of  fossils 
whether  a  rock  had  been  formed  in  fresh  water  or  in  the  sea  ? 

7.  Mention  any  facts  which  show  the  extent  of  sea-formed 
recks  over  the  earth. 


132  SCIENCE  PRIMERS. 


ORGANIC     ROCKS,    OR     ROCKS    FORMED     OF 
THE  REMAINS  OF   PLANTS  AND   ANIMALS. 

I.  Rocks  formed  of  the  Remains  of  Plants,  p.  56. 

1.  Explain  the  term   Organic  Remains, 

2.  Briefly  describe  the  characters  of  a  p'ece  of  coal. 

3.  Describe  the  way  in  which  coal  occurs  among  other  rocks. 

4.  What  is  the  nature  of  the  under-clay  of  a  coal-seam? 

5.  Whit  has  been  the  origin  of  the  coal  ? 

6.  Describe  a  peat -moss  or  bog. 

7.  What  is  peat  ?  What  have  been  the  stages  in  the  history  of 
a  peat-moss  ? 

II.  Rocks  formed  out  of  the  Remains  of  Animals,  p.  66. 

1.  WThat  is  the  origin  of  the  white  marl  commonly  found  on 
the  bottom  of  fresh-water  lakes  ? 

2.  What  is  the  ooze  of  the  At'antic  bed  ? 

3.  In  what  respects  does  chalk  resemble  the  Atlantic  ooze  ? 

4.  What  is  the  origin  of  limestone  containing  the  remains  of 
corals  and  slfells  ? 

5.  Give  from  various  parts  of  the  world   examples  of  large 
districts  and  whole  mountains  m-de  up  of  such  limestone. 

6.  Give  a  summary  of  the  geological  changes  represented  by 
the  sedimentary  rocks.     [Art.  161.] 

IGNEOUS     ROCKS. 
I.  What  Igneous  Rocks   are,  p.  74. 

1.  In  what  sense  has  the  word  "  igneous  "  been  used  in  geology, 
and  what  is  meant  by  igneous  rocks? 

2.  Into  what  groups  may  the  materials  ejected  by  volcanos  be 
divided  ? 

3.  How  are  the  rocks  of  these  two  groups  broadly  to  be  dis- 
tinguished from  each  other,  and  what  names  may  consequently 
be  given  to  them  ? 

4.  What  are  the  characters  of  lava?  [Arts.  165,  166,  167.] 

5.  In  what  forms  does  lava  issue  from  a  volcano?  [Arts.  168, 
169.] 

6.  Give  examples  of  the  occurrence  of  lava-rocks  in  different 
parts  of  the  world. 

7.  How  does  granite  occur? 

8.  WThat  are  the  characters  of  a  piece  of  volcanic  tuff? 

9.  How  has  volcanic  tuff  been  formed  ?    Give  some  examples 
of  its  occurrence. 


GEOLOGY.  133 


II.  Where  Igneous  Rocks  come  from,  p.  84. 

1.  What  evidence  is  furnished  by  deep  bores  and  mines  as  to 
the  temperature  of  the  interior  of  the  earth  ? 

2.  What  is  the  testimony  of  hot  springs  on  this  subject  ? 

3.  What  lesson  do  volcanos  teach  us  as  to  the  condition  of 
the  earth's  interior? 

4.  Briefly  describe  the  distribution  of  active  volcanos  over  the 
surface  of  the  globe. 

5.  What  evidence  is  afforded  by  dormant  and  extinct  volcanos  ? 
d.   How  are  earthquakes  related  to  this  subject? 

7.  How  would  you  explain  the  present  high  temperature  of 
the  earth's  interior  ?  Show  how  the  cooling  of  a  lava-stream 
helps  to  make  this  clearer.  [Arts.  185,  186.] 


THE     CRUST     OF    THE  •  EARTH. 

I.  Proofs  that    parts   of  the    Crust    have  been   pushed 

up,  p.  91. 

1.  What  is  meant  by  the  earth's  crust  ? 

2.  Of  what  materials  does  the  earth's  crust  consist  ? 

3.  By  what  kind  of  evidence  could  you  show  that  the  rocks 
of  the  earth's  crust  are  not  now  in  their  original  position  ? 

4.  Describe  a  raised  beach.  [Art.  193.] 

5.  What  evidence  is  furnished  by  raised  beaches  as  to  move- 
ments of  the  earth's  crust  ? 

6.  Why  do  we  say  that,  in  ordinary  changes  of  level  of  sea 
and  land,  it  is  the  land  which  rises  or  falls  rather  than  the  sea  ? 

7.  What  inference  may  be  drawn  from  such  ten-aces  as  those 
of  the  north  of  Norway  ?  [Art.  201.] 

8.  In   a  series  of  successive  raited  beaches,    which  are  the 
oldest,  and  why? 

9.  What  facts  have  been  observed  in  Sweden  regarding  re- 
cent movements  of  the  earth's  crust  ? 

10.  How  could  you  show  that  the  greater  part  of  the  dry  land 
'  has  been  raised  out  of  the  sea,  and  that  the  upheaval  has  been 

very  unequal  ? 

II.  Proofs  that  parts  of  the  Crust  have  sunk  down,  p.  101. 

1 .  Describe  a  submerged  forest. 

2.  What  conclusion  is  to  be  drawn  from  submerged  forests 
as  to  movements  of  the  earth's  crust  ? 


134  SCIENCE  PRIMERS. 

3.  Why  is  it  more  difficult  to  find  proofs  of  submergence  than 
of  upheaval  ? 

4.  What  facts  about  submergence  have   been    observed  in 
Greenland  ? 

5.  How  does  a  series  of  coal-seams  prove  former  submergence 
of  the  land?   [213-216.] 

6.  Mention  the  two  conclusions  as  to  the  movements  of  the 
earth's  crust  which  must  be  drawn  from  the  evidence  brought 
before  us  in  these  lessons. 

III.  Proofs  that  the    Rocks   of  the  Earth's  Crust  have 
been  tilted,  crumpled,  and  broken,  p.   107. 

1.  Besides   upheaval   and   depression,  mention  some  further 
changes  which  have  been  undergone  by  the  rocks  of  the  earth's 
crust. 

2.  Why  are  rocks  often  found  in  highly-inclined  positions,  and 
how  could  you  prove  that  these  were  not  their  original  positions? 

3.  Are  solid  beds  of  rock  ever  folded  and  crumpled  ? 

4.  What  are  faults  ? 

5.  What  use  of  faults  has  often  been  made  by  igneous  rocks? 

IV.  The  Origin  of  Mountains,  p.   113. 

1.  Of  what  materials  are  mountain-chains  built  up  ? 

2.  By  what  proofs  can    it  be  shown  that  many  mountains  are 
not  original  parts  of  the  earth's  su?  face  ? 

3.  What   are   lines   of  elevation,   and  how   have   they  been 
formed  on  the  face  of  the  globe? 

4.  Describe  some  examples  of  such  lines. 

5.  Briefly  state  the  nature  of  the  evidence  by  which  it  can  be 
shown  that  mountain-chains  differ  from  each  other  in  age,  and 
that  the  same  mountain  may  have  been  upheaved  at  successive 
wide  intervals.    [Arts.  231-236.] 

6.  Why  are  the  lowest  and  oldest  rocks  often  found  to  form 
the  higher  ridges  of  a  mountain-chain  ? 

7.  What   effects  are  produced  upon   the    external   forms   of 
mountains  during  the  general  waste  of  the  surface  of  the  land? 

V.  How    the    Rocks    of   the    Crust   tell  the  History  of 
the  Earth,  p.   121. 

1.  What  is  geological  history,  and  from  what  kind  of  evidence 
is  it  compiled  ? 

2.  Why  is  geological  history  necessarily  imperfect  ? 


GEOLOGY.  135 


3.  What  does  astronomical  evidence  point  out  as  probably  the 
early  condition  of  the  earth  ? 

4.  How  does   the  geologist  learn  which  are  the  oldest  and 
which  the  newest  parts  of  his  chronicle  ? 

5.  How  thick  is  the  mass   of  sedimentary  and  organically- 
formed  rocks  out  of  which  geological  history  is  compiled? 

6.  How  do  fossils  aid  the  geologist  in  the  study  of  the  history 
of  the  earth  ? 

7.  Give  some  account  of  what  is  meant  by  geological  history. 
[Arts.  253,  254.] 


LIST   OF   SPECIMENS    to   illustrate  the 
GEOLOGY  PRIMER. 


Sedimentary  Rocks See  Primer,  page  19 

1.  Conglomerate 21 

2.  Sandstone -  ...  9 

3.  Shale 22 

4.  Shale  containing  Plant-remains  (portion  of  a  Fossil- 

Fern)  44 

5.  Shale  containing  Animal-remains  (Trilobites,  &c. )  .  47 

Organic  Rocks       56 

I.  FORMED  OF  PLANT-REMAINS 56  . 

6.  Peat          62 

7.  Coal,  showing  stratified  structure 57 

II.  FORMED  OF  ANIMAL-REMAINS  ....  66 

8.  Fresh-water  Shell-marl 66 

9.  Ooze   from   bottom   of    Atlantic   prepared   for  the 

microscope 68 

10.  Chalk  with  Shell  in  it 69 

11.  Grains  of  Chalk  prepared  for  microscope       ...  12 

12.  Limestone  containing  Encrinites,  &c 71 


SCIENCE  PRIMERS. 


137 


Fossils 


I.  PLANTS. 

Plants  out  of   which  ' 

coal  has  been  partly  } 
formed. 
See  also  Nos.  4,  6,  and  7. 

II.  ANIMALS. 


13.  Stigmaria,  or  Sigillaria 

14.  Lepidodendron      .     . 


PAGE 

•     49 


61 


15.  Cup  Coral     . 

1 6.  Piece  of  Encrinite 


17.   Spirifer,  a  marine 


Animals  of  which  the  re-  ' 
mains  sometimes  form  thick  ' 
masses  of  limestone. 


shell    . 

See  also  Nos.  5,  8,  9,  10,  11,  and  12. 
Igneous  Rocks  

18.  Granite 

19.  Mica  .     . 

20.  Quartz  Crystal  j 

21.  Lava  showing  crystals  and  steam-holes    . 

22.  Volcanic  Tuff    . 


54 


74 
ii 


Substances  found  in  granite   .     .     1 1 


76 


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